My brother and I are selling a second home inherited in 2002. We don't know how much the cabin was worth then. How can we find that out to assess capital gains?

2002 or 2020 - sometimes there are typos? this assumes there was no estate return. there's no way to know if the property taxes for 2002 would give a proper valuation. in some places they are supposed to be FMV for others, it's some fraction.  another way is to contact a realtor familiar with property in that area to get a valuation. when you sell, and 1099-S are issued,  see that two are issued. one for each of you for 50% of the proceeds. 

A certified appraiser can give you a retroactive appraisal of the home as of the date of death of the person that you inherited it from. That is the best and most accurate way to determine the value, and the easiest to justify in the unlikely event that the IRS questions the valuation.

A real estate appraiser can give you a retroactive appraisal based on historical records.  You will have to pay for this service.  A real estate agent could give you a guess/estimate, but an appraisal would be better proof in case of audit.