It's not always obvious who a greifer is but it is possible to make some very educated guesses at who has destructive intentions.Through my personal observations, having banned a good number of griefers from my own server , these are some of the conclusions I have drawn.

Griefers names usually do not begin with a capital and are generally difficult to spell or misleading (e.g captainrunch) to prevent you from easily remembering them. I myself have never seen a greifer with a custom skin but I've heard of them, so if someone has a skin it is safe to assume they will not grief. Griefers are either not very sociable or exhibit almost anti-social behavior, as in they dislike other people instantly and such. This pertaining to their in game behavior not their real-life behavior which is irrelevant. If they do speak it is to either gain your trust (this is rare) or more likely to infuriate you.

More information is available on the minecraftwiki.net page about Griefing.

Post your own observations and thoughts. Remember if you see these things it doesn't mean the person is 100% a greifer and you should ban them. It just means you should keep a closer eye on them.