How to check/fix oil pressure gauge

I have a 2000 Bonneville 3.8 vin-K. My problem is that the original oil pressure gauge is at maximum when the engine is on. Once the engine is on, it will jump to the maximum, so I guess is the gauge itself. How do I really know if it is faulty? I neither have an external oil pressure gauge nor don't know how to use it anyways. I would like to know how to check the gauge, and if it is fixeable; if not, any guidelines on how to change it?

sending unit may need replacing.

I had the same on my 92 SSEI, I changed the sending unit and it works perfect