First, a Realtor will have access to the MLS. Depending on a number of factors, you may be able to trace the price history back anywhere from 10 to 30 or more years.
Second, most jurisdictions have tax records online. You probably can go online and find recent sales prices. Again, it'll vary regarding how far back you can go. Or, if you go down to the courthouse to look at the paper records, you can go back as far as you want.