It's hard to tell from looking, because often the consequences of cyanide poisoning don't show up for some time. The best way to avoid cyanide-caught fish, IMO, is to:
(1) Become familiar with where fish are frequently caught with cyanide. When you go shopping, ask your LFS where the fish came and avoid specimens collected where cyanide use is common.
(2) Become familiar with which species are often caught using cyanide, and especially avoid these specimens if they came from a locale where cyanide collection is common.
(3) Buy fish from the Red Sea or Hawaii and other locations where cyanide is not used.
(4) Buy fish from dealers that claim all their fish are net-caught.
(5) Be prepared to pay a little more $$$$ for net-caught fish from good locations (like Hawaii or the Red Sea). Remember, net-collecting takes more time and $$$ and if you're willing to pay a little more for responsibly caught fish, you'll be doing your part to help things change.
The cyanide leaves the fish's system very soon after it is caught. Pretty much the only way to detect it is to test the fish at the exporter's facility.
It is pretty much impossible to show that a fish was cyanide caught. Lot's of people blame "mysterious fish deaths" on cyanide as a result.
In the past fish coming from the PI has a high proportion of cyanide caught, but lately, Indonesian fish have a much higher percentage, so rules like "don't buy PI fish" are kind of useless.
I like deet's recommendation better. Buy fish from places you KNOW don't use cyanide. Plus KNOW what species of fish come from where. Be educated, that way if a dealer tells you that a royal gramma is net caught from Hawaii, you know he's full of malarkey.
eehhh... got you all beat - ONLY BUY CAPTIVE REARED OR RAISED FISH!!!
Actually, with the diveristy of specimins ORA and other's are shipping now, there really isn't much you'd have to/want to get that's wild caught. The Larval Rearing Programs seem to be getting into the more difficult to breed species rather well. I've seen a few brochure's on display at my LFS, and was amazed at the number and variety of things they are shipping.
Actually, one of the side effects of cyanide poisoning in fish is enhanced coloration. Another sign is if the fish is not eating. The cyanide can actually cause a reaction in their stomach. I never buy a fish straight away. I will ask the store to hold it for me for at least a week and I insist on seeing it eat first.