If you go wading after hard crabs with the dip net as described on the crabbing page, you will undoubtedly catch a few. My students or my family and their friends almost always do. You can actually tell on sight. The hard crabs are aggressive and move quickly. The thrash about with their pinchers grabbing almost anything they contact.
Soft crabs act soft.
They just roll into the net with their claws folded in to the body. No heavy duty thrashing. If you suspect the crab is soft, you can touch it with your finger or another object to verify your first observation. If soft, it absolutely cannot hurt you, but if treated roughly it will not live long and the body will almost fall apart. Reach into the net and take the crab up in your palm and place it in a container that is separated from the hard crabs whose random thrashing will destroy the soft crab.