I've read several places online in the past that a small glass jar or even a small glass set at an angle leaning up against the rockwork where you think they are living works well. Place some food in the bottom of the glass and leave it overnight, since they'll be most active at night. The angle (vs it laying flat on the sand) makes it so the crabs can't climb back out. I've never used it myself, but it's simple, inexpensive and seems to make sense. Seems like it's worth trying...
If you have any kind of shrimp, this may not work so well, as the shrimp will likely go into the glass, get the food, and swim back out.