The following steps are necessary at least in out of the way places where I buy fuel.
Use a funnel with a fine filter, but siphon off some into a clear glass jar. Get someone to inspect the jar carefully it should be crystal clear and when allowed to sit there should be no water
in the bottom.
I suggest if you have not seen water
in fuel add water some to a jar full of diesel
so you know what it looks like.
Keep filing while the fuel is inspected to avoid annoying other users (I always try to fill up at quiet times), but stop if the fuel is suspect.
Ignore the of the fuel vender who will claim their fuel is perfect and you are wasting your time.
If the fuel is good tell the vendor you are happy will buy from him again and recommend the fuel to your friends.
Turn on the polishing system. The tank has been stirred up and now is a good time to filter the fuel.
Seal the fuel port as well as possible. New O ring, silicon greece
on the O ring. My fuel port is on deck
so I seal them up with waterproof tape as a final barrier. My fuel capacity is over 1000L so I need to repeat these steps infrequently and can be picky where and when I buy fuel