I usually look at the manual to decide on a fuse to use, then i test it with a multi tester to see if that fuse actually work like i want ( that be come live with the turn of the key )
In my little cheap Suzuki car there was a lot of always live fuses, but in the end i finally found one to use for my 12 V expansion in the rear of my car.
For a dashcam or two i would probably have a go on any of the 10 A fuses, a camera only use about 0.3 A MAX so you can easy use a 3 - 5 A fuse for the dashcam side of the addon fuse.
I think the rule of thumb are to lay off the "smart" fuses, like the ones for airbags and other high tec stuff in your car, but there are a lot of other ones to try out, seat heaters and cabin light ( some actually tab into the overhead light )
Entertainment system circuit - rear heated window and what not.
If your item have a pilot light in form of a little LED that come on with the thing you can use that as a visual guide to see if its a always on fuse or one that come to life with the turn of the key. but otherwise you should be able to pick up a cheap multi tester for a few bucks.
If you have a soldering iron and a little wire you can also whip up your own test tool by using a 12 V light bulp as a indicator of when the fuse are live.
The most common error people make in hardwire are often that they dont use a proper place for the GND wire.
On youtube you can fine a whole host of videos on how to install a dashcam and hardwire kits for them, might be different brand cameras but really its all the same in that regard.