How can you tell if a disk caliper is going to fit on a given fork? there is an inboard width of a caliper that determines how far into the wheel it is set, ie. make or brake compatibility with the wheel.
I'm assuming the question is about physical interference between the caliper and the wheel, namely the spokes.
The fork isn't the determining piece here, because a given wheel and rotor is going to want the caliper to be located in a certain position relative to the wheel. In most (maybe all) cases where there is an interference problem, changing the fork wouldn't do anything about it.
The way this has played out for the most part is that some calipers have come out and been found to have issues with interfering with spokes on some or many wheels once in the wild. TRP Spyres were an example; when they come out in 2012-13ish they constantly had issues with running into the spokes of bikes people tried to put them on. I believe they fixed something in that design in a generational change because I haven't seen the problem in a while.
There are no measurements that brake manufacturers provide that let you predict the issue. It's become an unusual problem, even with large flange hubs. For the most part it's going to take a very wide design like the Spyres for a problem to even come close to being possible. The standard practice is to basically ignore the possibility of a wheel/caliper interference problem until it bites you.
There are three industry standards for disc mounts: flat mount (newest, mostly seen on road bikes), post mount (which looks similar, but with, well, little posts), and IS mount (which has a side plate the mechanism mounts to). This Q&A goes into more detail. These standards cover the disc's distance from the centerline and all that.
It's easy to visually identify which one you've got; once you know, you should be able to get parts that fit. There are adapters that let you convert in some cases (not all).