So the best thing for checking a body is something like this.
https://www.pure4c.de/accessories/measu ... -fdc-lightThat will tell you if the mount is good or if it’s out, how much you need to re-shim.
If you don’t have one the next best thing is to take a “known good lens” and use that.
That of course is hard to know without a lens collimator. But, the best way is to use something wider, as FFD focus is more critical the wider your lens. So you get a (say) 12mm fast prime that you know is good and set it to infinity, go outside and find something that’s at least a couple of hundred feet away and check to see if it’s in focus at infinity.
This only works if you have a “good known lens” and most people probably don’t have that.
To get a good known lens you use one of these
https://www.chrosziel.com/collimators/And then shim the lens so it is perfect for the FFD you set.
If you’re super nice to your local camera rental company they may let you collimate your own wide lens so you can then check your mounts.
JB