Don mentions "special tools". The special tool that I used was an engine compression gauge. Mine was the really cheap kind. A dial with a stubby metal tube. The metal tube has a rubber sleeve on it. You jam the gauge against a spark plug hole and watch.
To use it to measure rotary engine compression (Oh, no, that's a $600 tool), I took the Schraeder valve out of the end of the tube, so it would release the reading, allowing the odd triple chamber rotary to show its true compression.
To use it to measure K-bike fuel pressure, I pulled the rubber thing off. This left me with a 3/8" metal pipe coming out of the gauge. I had a metal tee left over from something. I used the tee and a couple of chunks of "normal" fuel line to tee this gauge in, at the aft end of the fuel rail, where the hose feeds to the regulator.
Hitting the starter switch should cause the pressure to kick up to 60 psi. When the engine starts, vacuum control on the regulator will pull the pressure down to 40 psi. Turning the engine off should leave you with 40 psi. How long you wait for it to leak down wasn't mentioned. Mine was still at 40 psi 30 minutes after I shut the engine off.
If you disconnect the vacuum hose that controls the regulator (mine comes off the rear throttle body), the pressure would go up to 60 psi. Applying some vacuum to this line (I sucked on it) should drop it down. I can get the pressure down to about 30 psi. This portion of the test might be performed while the engine is running, or by "bumping" the starter switch, which lets the fuel pump run for a few seconds.
When my regulator was bad, it would stay at 60 psi. The vacuum line had no control at all. Even at 60 psi, the injectors didn't leak. I let it sit for several minutes. The "normal" fuel hoses that I was using would swell noticeably at the 60 psi range, so it would probably be wise to use some FI-rated hoses instead.
The compression gauge isn't expected to be accurate in this pressure range, but I was looking for go/no-go, not tuning help.
With the engine running and the pressure stuck at 60, the bike actually ran most of the time. It was hard to start, but if I could get it started, and keep the RPM up, it was usable. And this was on a K75, with no feedback in the FI system. A K12, with feedback, might well adjust the injection to accommodate the high fuel delivery, but the injectors might leak down when stopped, dumping some fuel into places you don't want it.