I'm not one to break up a happy home. My fuzzy, ursine ears perk up when I see someone that's not already guilded. It's a potential recruit! I watch them play throughout the instance and also watch their social interaction with the group. If all goes well I ask them about their goals and try to ferret out a wee bit of their personality. Talking over vent helps too.
It's a little different when someone is already guilded. When I run PUGs with people repeatedly, I often ask them about their guilds. Many state that either their guild is small or that they're just "social." If it seems they're dissatisfied, and a good fit, I extend them a no-pressure invitation to come check us out.
I want to grow my guild with the right folks, but I don't want to be a poacher. Given the choice between all-guild runs all the time, and occasionally pugging, I will continue to group with strangers. How do you handle recruiting in pugs?