In the B707/DC8 era, neither of which had scavenge jetpumps to pick up water in the tank sump area, many operators used to drain a sample from each tank sump drain valve into a bottle (one for each tank) and test each sample for water with the F/E witnessing the test before departure. An operator I worked for back in the 60's regularly found large uplifts of water in their DC8's after refuelling at Honolulu as the fuel in the tank farm at HNL (a hot humid place) was being consumed so fast it did not have enough storage time to allow the water to settle before being used. On arrival at our colder main base after a 9 hour flight large amounts of water could be drained out after thawing. In fact the outboard engines suffered fuel filter icing as the freezing of all this water overwhelmed the fuel heaters and Douglas had to come up with a mod to improve the fuel heater efficiency (outboard engines fed from outer tanks which got colder than inner tanks).
Early 747's did not have a scavenge jetpump system and had a significant problem with the very large No. 2 and 3 main tanks accumulating huge amounts of water that affected the quantity indicating system (water contamination of the innermost probes, densitometer etc).