The best way to tell how much fuel you have left is what has been said above, just time it on your watch. i always forget to time it so i use the following method.
1. fill the tank untill it will take no more
2. fly until the engine quits and you have to bring it in for deadstick.
as soon as the fan on the front stops turning i know exactly how much fuel is in the tank (none) lol
thats why the irishman got killed flying his plane. he got cold and turned off the fan