One trick I've seen is to run a hose from an exhaust pipe of
a running
engine. The exhaust gases will drive out the gas fumes.

Even easier (and safer) would be to put down a hose from a compressor,
and let it pump air for a day, while at the same time trying to smell
the air coming out. I would think that if there is no perceptible
smell, including from an air sample from the bottom, there should not
be much risk of an explosion. But, better safe than sorry.

i

The exhaust idea sounds a lot better than the compressed air.

Haven't you ever seen the fuel delivery guy drop his cigarette but down the
hole at the gas station? No air down there to support combustion...