I would personally start with an Airthings Corentium to measure your radon ($136 on Amazon right now). Immediate results (once you’ve gotten a weeks worth of measurements) and the benefit of constant measurements going forward.

Or you could get the manual at home radon kits for $20-ish where you have to mail it to a lab and they send you the results after 1-2 weeks. Last time I had a company test for me, they just used one of these kits anyway.