No sure fire way - we caught our nasty crabs "accidentally" - heck, I fell into the tank catching the first one (don't laugh!).
As far as the Mantis, if you know which rock it's in, remove it. Place it on top of a small bucket, inverted inside a larger bucket. Place a couple inches of tank water in the larger bucket. Douse the rock with some plain, non-flavored seltzer water. The Mantis will jump, and plop into the SW
in the bottom. That's how we got our Mantis.
They do make traps - and crab traps as well - never personally used one.