The Fox Now app is free to download and install, but you’ll need a cable TV subscription or an account with a supported over-the-top service in order to actually watch anything.
The good news is that plenty of supported platforms, including Hulu, YouTube TV, fuboTV, and Sling TV, offer a free trial period (usually one week). This means that you can sign up, stream as much as you like, then cancel before the trial ends to effectively watch Fox Now for free.