Recently pulled Magebane and Theifbane from boosters, and promptly tossed them into a mix of treasure chest for a game.
The problem is, as I was clicking to find what trap a chest had, I wondered what the odd smudge between some of the traps were, then looked closely and saw it was the Dragon's Gate symbol.
So, you can actually tell which chests are mimics before they ever get opened!
Which brings us to the problem of...what can be done so you con't actually know which chests are mimics?

We really, really try to not look at the chests when we pull them, and that seems to work pretty well. This also keeps us from judging the chest based on its number (in original dungeons, the higher the number, the more deadly the chest).

The 'banes are awesome, and really fun...I think that it is up to those that play to be reasonably fair about the whole thing, and not microinspect the chests before trying to disarm them.

One thing that you can do is put a "place holder chest" on the board. This chest is used for the trap. When the trap has been sprung/disarmed and you want to open the chest, then reach into a bag or something with the chest pool in it and draw out a chest. This chest is what will be used for the treasure in some cases "the spawn"!

Wouldn't that cause problems for you since generally the better chests have nastier traps on the dial? You could go through a really nasty trap then draw a really crummy treasure chest! :D

I suppose you COULD, but if you then limited it to Pyramids chests, they are much more well rounded.

Also, the randomness of the Dungeon would be enhanced by Korak's suggestion, which is fine by me....

Well, last game of Dungeons I played we drew 4 random chests out of our "box o' chests" which contains one of everything we have (so nearly all the dungeons and pyramids chests, plus the two mimics).
Somehow, we got both mimics.
Each of us opened one of them during the game, and neither of us realized it was a mimic until we opened it up, stared blankly at it, and said something to the effect of "What the heck kind of treasure is this???"
:D
I should have been more prepared since she found Magebane first and had that response, but then I figured there was no chance we'd get both of them and had the exact same puzzled response.