I just bought a new house (first one!) and the existing
water heater is over 15 years old. My father-in-law has
a 50-gallon gas water heater that he only used for a few
months before upgrading to a bigger tank because he
upgraded his bathroom...

Question: If the 50-gal water heater was emptied and has
been sitting in his garage for 2 years, are there any
problems that could exist or manifest over the years in
an empty tank?

I'll check the anode and pressure valve, but any other
major things to look out for? Any quick & dirty tests I
can do when I first install it to make sure it's good
for the next 10-15 years?

Thanks!