Four years ago we had our furnace checked and some maintenance done by a
repairman who worked for the company that gave us our oil. He said our
fuel tank was getting thin and we ran the risk of it rusting out and
therefore spilling fuel oil all over the floor (the tank is inside our
house). He said we had maybe 2 more months and should get it done.

This was four years ago. Our tank is still holding up fine so far.

Last week, we had another guy come in and do maintenance on our furnace
again. Not surprising to me, he also said the tank was getting thin and
we should have it replaced within 2-3 months.

How can I trust these guys?

Sure, I have no way to know if they are telling the truth or just need
some more income coming in.

Or, if I don't act now, will I have a flood soon in the furnace room?

How can I be sure this guy is telling the truth?

Thanks