As for a 15 bar pump. No, pressure is pressure... if the pump is going at 15 bars then the group is also pushing water out at 15 bars. One way to tell this to install a pressure gauge on a portafilter. Now being there are several different types you might have to come up with different ways of adapting your pressure gauge.
The other way I test pressure is from a fitting I designed. On machines like Jura Capresso you have plastic lines that feed water to the group. They usually have a John Guest fitting so with a little time and money you can adapt John Guest fitting with a few bigger fittings until it is big enough to attach a pressure gauge, thus allowing you to test the water pressure at the group.
The reason I made the fitting was for a machine that had a Procon pump, but you could not attach a pressure gauge to the wand. The wand was all one piece and you could not take off the spout. I needed to make sure it was set around 9 bars. After wasting 2 hours it tested about 9.75 bars maybe 10. But now I can test it in 30 seconds, it’s a nice tool to have.