Rob, you're welcome. As an example, all the blade stamps are the same, all have brass guards and end caps, all have the same type of sheath, most have stag handles but a few have ivory. The blade size and shape, including the choil, are going to vary a little; the handle sizes (diameter) will vary a little too, but not much. Look for a visually well balanced knife.
Also, I did a little work for you - all the RMK Mini #25's currently listed on eBay as of the time of this posting (like, this minute, but maybe not the next) are legit, as are all those shown in the completed auction listings.
Get to know the knives, to the point that you'll know if something just doesn't feel right (JDFR); if it doesn't feel right, it probably isn't. If you need more info, just ask...here or via PM.
Shel