Canada

This is a good start to learn about it

https://cancer.ca/en/cancer-information/reduce-your-risk/know-your-environment/radon

These are approved devices

https://c-nrpp.ca/approved-radon-measurement-devices/

There are cheaper options where you just leave a test item in a specific area for 90 days and then ship it to them to be analyzed, or there's more expensive electronic options that give quicker readings. It's possible a local library or other community spot might have electronic ones available to rent/borrow as well.

Generally you want to measure over an extended period of time as the amount of Radon that is present can vary over time. It is also worse in the winter as we keep our doors/windows closed more often giving it a better chance to build up, and measuring in the basement is best as thats where it would build up the most.

If your home happens to be in a spot where there's too much radon, you'd have to install a system that exchanges the air between the inside/outside at some frequency.

You probably won't find a house with 0 radon, it's just a matter of how much is present and if it's too much.

My friends mom died from the (or one of the few) lung cancer you can get even if you were never a smoker. Her sewing room was in the basement and they think it may have been due to above average radon levels.

1 in a million chance or something, but apparently more likely with exposure to some things, including radon.

We're in Alberta and used this site: https://www.homeradontest.ca/

It was cheap and easy. Unfortunately our results were right at the warning level, so now we're trying to figure out what to do.