Well it isn't rocket science. Basically you just look at the tank and the fuel.
First step is just to visually inspect the inside of the tank. Best to do with tanks
almost empty so if they're full pump the fuel into jerry jugs.
Does your fuel tank
have an inspection
port on top? Take it off and look inside with a light. Unless you have a fairly large inspection
hole it's hard to shine a light into the hole and see at the same time so I made a small inspection light by soldering a 12V flashlight bulb onto two wires and drop it into the tank. DO NOT DO THIS WITH GASOLINE TANKS!!!
Then get a small pump, whatever you have around, and suck the fuel from the very lowest part of the tank into a clear bottle or jar to look at. I had to tie wrap the pump hose to a stick to get the hose into the low spot. You should do this with the boat at a very calm spot where it isn't rocking in waves or wake so any crud will settle into the bottom of the tank and be easier to see and remove.
If the tank is coated with black grunge and the fuel full of goo then time to get serious.