I know I have to have a preinstalled place to bolt the caliper, but how can I tell by looking what kind of braze-on I have on the rear end? Since I'm having to replace my back rim (Wobblenator 5000, btw) I have an excuse to check.
Look at the pictures, particularly the close ups, of the various mounting bosses on the frames and forks.
There are some retro-fit mounts out there, but if your frame is not designed for disc brakes from the drawing board few folks would recommend converting to them.