If you're not aware, you can just tap search and the description on a chest gives it away.
I'll also add that I don't think that this feature is a good idea from a game design standpoint. That is, it's something nonobvious that players need to learn, so it increases the complexity of the game, but what it adds in terms of gameplay isn't an "interesting decision", since a player should always check -- just adds drudge work.
One wants to only add complexity to the extent one must to create interesting decisions for the player. Keep it easy to learn, but hard to master, minimizing the actions that are no-brainers.
Linley Henzell, the Crawl creator, has some quote along the lines of "if it isn't an interesting decision, it shouldn't be in a game".
I think a better route would be one of:
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Remove the existing game-resource-cost-free mechanisms that identify a mimic. That is, as far as I know, searching, waiting for the animation on the mimic to run, and apparently this wand thing, though maybe the last is a bug. Those can't really be interesting decisions, because there's no tradeoff (well, maybe with the exception of the player's time for the animation route, but I think that games that make one trade off a player's time doing something boring aren't a lot of fun, as "being boring" isn't something that one wants a game to be).
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It's possible that undetectable mimics are fine on their own, if it makes players weigh the risk of opening a chest at low health or something.
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If having detection of mimics in the game is desired, then the cost of any method of detection should be significant enough to make considering doing so a real decision.
