I see n00bs all the time.
You know how I spot them?

1. Trying to kill a creeper by running up to it and keep moving in front of it will the creeper is getting knocked back.
2. building a house with wood logs.
3. not knowing you can put a pumpkin,creeper head,Steve head,zombie head,and a skeleton head on your head. (YES you can do that n00b -_-)
4. Challenging a mod to PvP
5. and last but not least not knowing how to find a Minecraft server (Just look up minecraft servers on google n00b!)

Thanks and let me know some ways you spot n00bs :)