Either a testing company can come in for a hundred bucks with a continuous monitor and educate you about it...or you can get a kit yourself from the local dept of health, hardware store and big box store.
I own some machines called femto-techs. I consider them the best for my use. They continuously read the radon levels in the home at that time period.
Get the pro in to do it is my suggestion. But if you do not, make sure the house is closed up like it was a cold winter outside during the test. And no testing during any storms, so check the weather or leave it to test longer. This is where it gets tricky.
I went to radon training and had to take a test, that's why I say use the professional. There is so much more going on.
Where is that radon guy??
and go to radon