Although you say a GM shouldn't play legits, I find that unfair. Slightly. I like GMing for servers but I also like to take a break from time to time and just level. Now, I dont have ingame commands like @item or anything really. All I can do is Mute. Haha, and turn into random stuff. I'm limited on the events I can run and I cannot select my own prizes, they are all pre selected because of the rank I am.
I am honestly glad I am one of the lower ranking GMs, I dont need @item. I would much rather hunt for my own stuff.
But I agree, you should just interview them, try them out. Let them just be a 'support' GM or something, let them answer questions, see if they are knowledgable. Honestly I dont think any person should have @item or serious Commands unless its the admin. But if you have a corrupt Admin, well...then it doesn't really matter I guess.
There is no real way of telling if someone wanting to be a GM, or who is already a GM, is planning to be corrupt or do corrupt things. Just keep your guard up, if they keep asking for powers or it seems suspicious, it probably is. o_o;