Years ago, I was working in a biochem lab, and helping a state senator's staff draft legislation. The state capitol building was classic granite and marble. So one day, I brought in a Geiger counter. Everyone freaked ;)
But yes, much granite does contain substantial K-40 (as do we) and some (probably less than 10%) is a substantial source of radon. Also thoron, but it's too short-lived to accumulate. You can buy test kits designed to measure radon production rate from surfaces. The radon gets captured on activated charcoal. You return the kit(s) to the provider, and they send you results.