Hard to tell until you physically see it and know what you are seeing.
Pistol shrimps and Mantis Shrimps make next to identical noises...
Mantis shrimps are NOT the machines of death destruction and mayhem people make them out too be....having said that they are capable of extreme destruction! Each mantis has its own personality, if it IS a mantis shrimp you likely have a smasher as its rare to get spearers as hitch hikers being sand dwelling stomatpods, IF there is a mantis shrimp and because of the high chance it will be a smasher, your fish are "virtually" safe, it is very rare for a smasher to actively hunt down a fish because they lack (for the better part) the required tools to kill fish, in short the smasher knows what his "weapons" are good against so any hard bodied critters like molluscs / crustaceans will be the first things you start to see disappear.
Mantis shrimps are extremely intelligent, so it WILL figure out your "routine" when you feed fish, when the lights go out, when the lights come on, more importantly are the fact that they are extremely hardy and will survive off the small bits of food that your other inhabitants do not eat / float into the rocks, once it understand when lights out are if it is hungry enough it will start to hunt at night and THEN pick off your fish while they sleep.
Would it not be so easy and KOOL to be able to do what those asians do with birds and fishing, where by the asian ties a rope around the birds neck, then when he releases the bird into the water it would "duck dive" catch fish but be unable to swallow them because of the rope and hence well catch fish for the fisherman! Recreate that with a rope around a larger mantis shrimp so you can at least get it back out quickly while it hunts down the smaller mantis you want out of the tank! LOL
To catch a mantis you must first BECOME the mantis =) because if there is ONE thing in a tank mantis shrimps will not tolerate...its another mantis shrimp!
Hope this helps...