The right tell is no tell
Nothing causes a gank more problems than a blown cover. A ganker wants to remain undetected up until the moment he charges out from the fog of war, and many gankers want to take it one step further and deliver a crowd control from their hiding place before they are revealed. If the ganker's position is known or even suspected, the victim can pull back to safety before the gank happens. This wastes the ganker's time, and while it may lead to a bit of lost gold, he's probably out more than the victim is.
The most obvious way to reveal a ganker is wards. A strategically placed ward to reveal a ganker's approach is the hallmark of mid-level play, and it's one of the reasons live audiences still celebrate the destruction of wards in professional games. Destroying a ward reveals the person clearing it, but it costs money to place them and gives some back to the ward clearer. The moment that ward goes away, uncertainty reigns. Does the clearer leave after destroying the ward? Or does he (perhaps foolishly) move into position for a gank? As the victim, you don't know, and until you can replace that ward, you won't know.
Even without wards, a gank can be telegraphed in other ways. For instance, if an enemy Leona leaves her lane, she has gone somewhere. She might be returning to base to shop, but if she was relatively full on health or mana, she may be roaming. She could be in your jungle right now with the enemy jungler, looking to kill your jungler as he goes for his buff camp. She could be in position to gank mid. Just seeing her missing is a tell. If she's gone for a long time, it becomes hard to judge what she might be doing. You can't completely surrender your lane just because the enemy support went MIA, but you may be putting yourself in danger. She could be dropping a Solar Flare on you right at this very instant. If you leave to go support another lane by ganking, remember that your absence telegraphs that something is wrong. Try to avoid showing that you're going anywhere but back to base, or give tells that suggest you leave your lane regularly to ward so it won't be quite so obvious when you decide to gank instead.
The most telling sign of a gank is a change of lane behavior. If a normally passive enemy goes aggressive for no reason, it is a good sign that you are in trouble. In Dominion matches, I can often predict a bottom lane gank simply due to this. Often the person visible tries to set up the person ganking, and that can be fine. However, if it's an abrupt enough change, it can be a sign of an obvious gank. The key tells are aggression -- particularly of the type that will make a gank easier, such as CC -- and used in ways that aren't normal for the person laning. This could be used to throw off a smarter player, if you've done similar things followed by a gank in the past. However, it's sort of a gimmicky thing to do. In general, try to avoid changing your behaviors until it's already too late for your enemy.
Practice, practice, practice
The best way to execute ganks is to practice and time them. If you know how long it takes for you to activate Bear Stance as Udyr and charge from the brush into the lane, you can have a good idea on how far your opponent will be able to run before you arrive at him. Skillshot-based ganks such as Sejuani's ultimate need lots of practice. You don't want to miss the stun at a critical moment.
Practicing ganks can be useful even if you're playing against the computer. As long as you're working on technique and learning something, bots are an effective training tool. They won't respond like humans, so the goal isn't to simply execute a gank. The goal is to execute it as perfectly as possible and give yourself an idea of how much time you need to get things under way.
Stagger crowd controls
This is the easiest advice to give and the hardest to follow. Don't stun someone who is getting knocked up or stunned already. Avoid stacking multiple slows if those slows have cooldowns. You want to make sure that your target spends the most amount of time disabled or unable to run as possible.
In practice, this is rather difficult because you have no control over your allies. Go the extra mile to look for things like the animation of crowd control moves and react appropriately. Some, like Leona's Shield of Daybreak, are hard to see at a glance. If you have multiple crowd control effects, fill the gaps between them with autoattacks or other damage. Leona, for example, can snare with Zenith Blade and stun with Shield of Daybreak and Solar Flare. It's a bad idea to snare followed by an ultimate and a shield bash. Instead, she should snare, land a basic attack, followed by a shield bash, then lead slightly with Solar Flare to hit the opponent as he runs. It's also possible to lead with Solar Flare, then walk in and bash, then use Zenith Blade to catch your foe as he runs. Keeping your foe disabled for as long as possible gives your allies the most time to leverage the maximum amount of damage against him.
Don't try too hard on a failed gank. If you're spotted and the enemy withdraws prematurely, don't stick around. Do something else. Your position is already compromised, and the enemy already knows where you are. As a ganker, you're supporting your allies. If you are detected, you become a liability because the enemy could gank you.
I played against a very aggressive jungle Fiora once, and I was determined to not allow her to get kills. When one of the lane wards spotted her, I pinged her as she moved in for a gank. Her target lane -- the bottom lane in this case -- moved up enticingly while my mid left to head toward her. When I Flashed over the wall to Bear Stance punch her in the face, three of my allies converged quickly on her and quickly ended her day. We forced the bottom lane back and took the dragon as a trophy.
If you suspect your gank isn't going to work, don't go for it.
Use information and intuition
The most important skill of a ganker is knowing the battlefield. If you can predict the movements of the enemy jungler, you can beat her to the punch. In the Fiora situation I mentioned above, I was fairly aware of where she was during her jungle route, partly because I was aggressively counter-jungling and knew the timers on her spawns. I had a good idea when she would gank, and all I needed to do was guess where. Because our bottom lane was pushed up a bit, I guessed she would go there, and we were rewarded for my good (perhaps lucky) guessing.
Warding and vision of other kinds (using teammates as wards in Dominion, for instance) can provide you with the information you need, but sometimes you need to go with less than perfect data. In one case, I saw an enemy Rengar cross through the middle area of the Crystal Scar slightly, moving towards my team's side. I reacted immediately and pinged our bottom turret. Sure enough, Rengar came out of the jungle a few moments later and began capturing. Luckily, our team had responded to my ping and managed to repel him. Unfortunately, it caused me to die since two people responded to my ping, leaving me outnumbered at our top turret in a 1v3.
Intuition comes from taking small bits of information and extrapolating. In that case, I had seen Rengar move through the middle area in a very obvious way. Sometimes you're given less information than that, such as an enemy walking through a Shaco box in the middle brush or leaving the mid-top lane area for the health relic and not returning. If you see an enemy go MIA, ask yourself: Why?
If you don't see an enemy go MIA, but he is missing, ask yourself where he might be. I've had a lot of Dominion games where I could count on a particular foe to be camping above the top turret every time. Other times I would not see an enemy player for 20 seconds and ping my bottom lane to get back because I felt a gank was coming. Sometimes, if the situation is favorable, I'll move myself and/or allies to cut the ganker off and kill him -- all based on a hunch that he was ganking simply by being MIA for 20 seconds.
I'm not perfect at ganking by any means, and it's a very situational skillset that is hard to teach. As you play more, you'll get better at it. Good luck and have fun!
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