I belive people use seltzer water and add salt to get it to the right SG. THe bubbling will make the critters go crazy and shoot out of the rock.
The hitchhickers you keep depend on what you want to do in your tank. If you plan on going full reef most crabs are best left out of the tank. Snails and shrimp such as pistols are usually okay to add. Other worms and little bug type critters are usually safe as well.
Place the rock in a container of matured saltwater with a specific gravity of 1.035 - 1.040. Everything living will leap off the rock like its on fire. Wait one minute and remove the rock quickly and inspect for any bristle worms still halfway out of the rock. If you want to remove them have some needle nose pilers or tweezers on hand. You can now sift through the bucket and pick what to keep and what to give a 21 flush salute.
Here's to swimmin with bowlegged women.
-105G SPS Reef with 30G Sump/Fuge Combo
-30G LTAnemonarium with mated porcelain crabs, 10 sexy shrimp, and 2 pink skunks all tied into the other 135G in my system.