I cannot claim this as my idea, thanks goes to Alan off yacht Melody, but I thought it such a good idea that it needs putting on the forum.
The idea is to have a permanent method of testing the fuel in the tank that takes around 3 minutes to complete.
1. fit a pipe in the top of the tank that goes to the lowest part of the tank, something like an eberspacher take up pipe, or similar.
2. Fit fuel pipe from take up pipe to a hand bubble type pump.
3. Fit clear plastic pipe from exit end of pump to a clear container
4. Hand pump fuel into container to test.
A rubber plug can be fitted to pipe when not in use.
Its simple but very effective.
Odysseus