Hi,
Persisitent clicking noises, without the carnage of dead small molluscs, missing crabs, etc.
That would be my clue.
At the small sizes they usually come in (or sneak in), it is next to impossible to distinguish between Pistols and Mantises visually (if you ever spot them at all), unless you capture them and examine them closely.
Most pistols have one large, fat Popeye-the-Sailor-ish claw opposite one runt claw. Mantises have either a pair of Praying-Mantis like spear-claws, or else a pair of hammers for bludgeoning prey.
Pistols are NOT a pest. At the sizes we are talking about, they use their pistols to stun small worms and crustaceans to eat, or to shoo away threatening fish or crustacean predators.
Mantis shrimp more closely fit the bill of villain, and are bold, efficient predators, especially once they attain some size.
I have yet to hear of a pistol getting very large in a hobbyist tank (they likely fall victim to one predator or another), whereas mantises nearly reaching lobster-size have been reported.
Maybe you can scoot around the 'net for pics of either.
hth,
horge
