How to tell if a crab is good or bad

[canadian_eh]

I was wondering if there are any general rules to tell you if a crab will cause problems in a reef tank? Like the claws or some behaviour. I caught this one last night cause he looked pretty mean.

TIA

edit: you can't tell from the pic but it's bigger than a quarter.

 

[Bearfan]

If it's a crab...it's bad.

That being said, I know I still have a couple of porcelain crabs that donut mess with anything. Everything else I know of is in a separate tank.