Checking fuel pressure with a tire gauge???
 

I've read on other forums that it's possible to check fuel pressure on my M54 with a tire gauge on the fuel rail schrader valve. Is that true??

If so, how exactly does one do that?

I wouldn't do it

on many cars there is a quick connect style fitting off the fuel rail
some other cars its a threaded fitting
I have never checked what my x5 has

but I wouldn't use a air psi gauge for a liquid application, the gauge will mess up from the inside

ontop of that a tire gauge is not a 100% sealed conction so it will mess with your diagnosis unless you can thread it to garentee no leaks than by all means try it out