Hi, I got a cold cylinder on a '91 Suzuki GSX R750cc, when sitting on the bike its the last cylinder to your right. I noticed it cause my idle was rough and started a light smoke with a strong gas smell. When I touched the exhaust pipes (very carefully to not get burned) at the cylinder, one of them was cold. Anyways, what would be the best way to check if the plug or the wire going to the cylinder is still good ? Or could it be somethimg else ? By the way, I appreciate your time.

1)You have two coils servicing four spark plugs. This is called the wasted spark system because the spark plug fires on the compression and exhaust stroke. Therefore on one of the coils , one spark plug is firing and the other is fouled. I would guess that you have a bad spark plug cap on that cylinder(about $25). Replace the spark plug cap and the spark plug and your problem should be solved.
2) You have low compression on that cylinder and need a rebuild.you may want to pull the plug and do a compression test.