few options available.
float on a rod or similar affair. one type uses the arm on a pivot like most car guages... some use a very coarse thread and the float is prevented from spinning as it rises...the thread is attached to the guage pointer. this was common on the cap used on (OLD) B&S lawnmowers.
another one is to just have the rod coming out of a gland in the top of the tank... up is full, obviously. very common on small aircraft, cus its virtually fool-proof.
cut a slot in the side, reinforce it with a flange, and then clamp a piece of polycarbonate over it. more suitable to a custom made tank...
and of course, a strip of polycarbonate (i say that only cus im sure its fuel proof) cut into a sharp pointy triangle. polish the top short side and stick it out the cap etc. when just the tip is in fuel...you only see a narrow line in the middle. when the whole things submerged...the strip fills up. another mower way of doing things...victa i believe. they had the strip with parellel sides, but each 5mm it narrowed down a few mm, gave it a sort of "notch" effect (ie, 1/4, 1/2, 3/4, full...) rather than a smooth transition from full to empty which a triangle will produce.
or take the lid off and peek inside