there are a million ways to remove mantis shrimp but the one i use is a luxury method, as i have alot of tanks at all times I will remove all cared for livestock from the suspect tank, and I will then starve the tank untill the bugger comes out looking for food, then i usually put in a mantis trap of sorts baited with some fresh fish, works like a charm for me, but mabey not practical without many extra tanks.
As for post mortem, I have been wholesaleing for a few years and have seen many dead fish, most of the time I can tell by looking at the dead fish what it died from but sometimes I do open the fish up and find out what killed them. I usually move larger fish which makes this easier. If you suspect a fish is cynide caught open it up and look for signs, usually the organs will be jellied. If you do find a cynide induced death, take the dismantled fish back to the LFS you got it from and let them know that the fish died cuz it was captured with the use of cynide, hopefully they will look into it and change suppliers(ask what there doing about it) if they do nothing let them know that you and all your buddies will no longer purchase fish from their store, if you dont do this you yourself are encourgaging the use of cynide fishing. There are many sites that explain symptoms of a cynide caught fish, check a good search engine...