They're not scams, but they're not really worth it either. IQs fluctuate quite a lot throughout a person's lifetime because they are still subject to a ton of environmental factors. It's best just to assess your skills and try to better yourself. Unless you have an insanely high or low IQ, it won't ever really matter. Besides, it's not really something you want to put on a resume or brag about unless you want to get laughed at or you're applying to Mensa.
If you're just curious, free online tests can give you a ballpark estimate, but make sure to find one with some officiality. There's a lot of scam IQ tests that try to bait you into giving info and money. I believe Mensa offers one.