There is a unit thats available that will check fuel level, and turn on a light, but it would be hard to see on the airplane during flight. I don't remember who makes it, or how reliable it is, but I did see one on the market in the last year.

The easiest and most reliable method is to time your flights. Once you do a number of flights and work out the time, its very reliable and accurate. 15 minutes on my 40% Extra, and I always land with a 1/4 tank for reserve. On my small Somethin Extra, 12 minutes and I land with just a smidge leftover. I never deadstick unless I run past my timer.

Wish I could remember who the manufactuer of the fuel gauge was, but maybe someone who has a good memory will speak up