First, other than trees at swamp, mimics chests give themselves away by shaking periodically. Just gaze at chest for 5-6 seconds or so before attempting to interact.
Second, it always small chests / safes that can be mimics. Iron chests / large safes are always normal.
Third, mimics is a different type of curio and tend to get a different reaction from girls. Girl shied away from chests or constantly raised false alarms, but is eager to open it now? Most likely a real mimic this time.
Fourth, collect cursed items and make them infinite. Items that were made infinite stop spawning on field in non-forced way.
Fifth, get a girl with 17+ wisdom stat and listen to her opinion. She will identify mimics at first glance.
Sixth, read bottom part of
that post.