I'm skeptical that CPAP machines frequently lose pressure over time. However, you can buy a manometer for under $100 from Supplier #1 in the supplier list. I would get the model with the gauge, not the water column version.
You can do a very rough pressure check by sticking the end of the hose into a bucket of water and measuring how far down into the water it will force bubbles out of the end of the hose. 10 cm pressure means it will blow 10 cm deep. Turn the machine on, stick the hose deeply enough that no bubbles come out. Slowly raise the hose until bubbles start coming out. Don't get water on the machine itself. If you try this with a glass of water, it will tend to go everywhere. It's a very crude measurement, but it gives you some idea.
It's not at all uncommon to have your pressure needs change.
Your machine will give you some data on AHI and pressure usage once you learn how to read it, even without software. Figure out your AHI, pressure settings, and 90% pressure.