Here is how you can tell... this works every time.
Grab an iron (like for ironing clothes) and turn it at the highest setting... more on this later. Then paint your finger brownish green (think snail camoflauge) find a shell small enough to fit snugly on the painted finger. Now stick your finger in the tank and slowly move it around the tank on the substrate. The trick is to mimick a snail so you will have to move it pretty darn slow. You are trying to be a snail here, and what do they always do? Eat of course. You should make obvious eating noises, you know, the kind you make when your wife is not at home. You could also throw in the occasional "yummy, algae!" Try to make your way close to any potential caves, some mantis are plain lazy and will only come out if you are just outside their burrow. You may also throw out a few cat calls to passing lady snails - just to add to the realism and further convince any possible mantis that you are indeed a snail. Now for the actual detection part. If you, at any time, feel a quick tug on your finger, don't panic. Visually check your finger. It has been taken clean off at the second knuckle, right? Yeah, Mantis are so quick and razor sharp that you don't often feel the pain from their attack for a few seconds. Remember that iron? Yep, grab it and use it to cotterize your stump.
So that was the first step - detection. If you are lucky, I may follow up with directions on what exactly to do if you do find a mantis. I will also tell you what you can do to keep that sneaky mantis from stealing your identity - remember he has your fingerprint...