Depending on my experience, visual inspection for FRP equipment is best and effective way to assure the equipment integrity, of course, it's better to do inspection by relative expertise.
normally, spark test is also a good method to evaluate the integrity, but if you do in service equipment inspection, you should note the spark test, i think visual inspection is the only way.
And you can find out API 12P Specification for Fiberglass Reinforced Plastic Tank, I guess you can obtain more thechnical hint.