Can drain the fuel over your fingertips, you'll feel the difference between fuel and water.
Otherwise (not the best, don't really recommend it, but it works) tip the drain over a tyre. Water beads and fuel seems to 'absorb'. They look completely different.
If in doubt, don't bother.
If you end up in some really bad stuff, do a 180 and try a different way.
If you end up in cloud/reduced vis (and maybe you shouldn't be there being VFR) and if you decide to bash thru it, at least do it at the LSALT.
Carry EXTRA fuel on top of what is needed. An extra 20-30min wil go a LONG way, and it gives a comfort factor as you know "I've got heaps!"