No real signs other than sudden death or rapid deterioration but even then, it could be something else. Only trace amounts of Cyanide stays in the fishes system (organs) and cannot be tested for until the fish is dead. You also will not be able to test at home. The fish needs to be blended up and you need very highly sensitive equipment to pick up the trace amounts of the poison left in the fish. Not something most people can test for and not something labs are going to bother testing for.
I have found that fish I suspect are dying of cyanide get "bruises" which I attribute to internal bleeding or hemoraging of internal organs such as liver. I don't know for sure though.
Usually the fish will be fine one day and almost dead by the next. They stop eating, loose weight rapidly and often start to swim erratically or can't swim at all. This is all just from experience though and may not be linked to cyanide. I am pretty sure it is but like I said, I could never be certain.
At MACNA last year a speaker (sorry don't remember who but he is well respected in the industry) claimed that 2/3 of the fish in the industry are from the Philippines and that approx. 3/4 of them are cyanide caught.
You are much safer going with Aussie fish and in the past Indo fish. But cyanide is starting to catch on in Indonesia now too.
Angels, Butterfly, Mandarins, any cryptic wrasses, some gobies, and most hard to catch fish are often cyanide caught.
If you have ever been on a dive among these species of fish, you will know that when you get too close, they dart into the coral heads and would be impossible to catch with a net. On the other hand, some triggers, puffers and even tangs I have found to be incredibly friendly and would not take too much effort to net catch...especially if you have some food for them
