most thorough way of checking -- remove pump, being careful not to bend float lever. clean and check pump casing. Spray well with penetrating oil (GT-85, Plus Gas, not WD40), tank mounting bolts beneath car.

Drain tank as best you can, remove tank, drain completely.

Get a litre or so of methylated spirits, slosh around in tank, stain and continue sloshing, drain.

Meths is great as it absorbs water, but is combustible (is generally a 50/50 methanol/ethanol mix). Refill with fresh petrol.

Report back.