Oil Pressure Guage or Sending Unit

[N29sailor]

My A4 normally runs with 50 psi when cold and at 40 psi on the oil guage when warmed up and cruising. Today I started the engine and found only about 2 or 3 psi. Question: How to tell if it is the sending unit or the guage? What voltage would the sending unit put out to read about 40 psi? Anyone know? Thanks.