If the gages came with the engine package, a shop manual will let you know
how to test both the guages and sender unts.

This will usually involve disconnecting sender connection on the gage,
turning the ignition on and verifing that the indication is (usually) at
minimum. Then the send connection is jumpered to ground and the indications
should be at maximum.

The sender units may be slightly more complicated, but with a thermomerter,
digital vom and some inexpensive pressure gages it easy to verify almost any
sender and gage combination.
For example, on an old Mercruiser:
oil pressure
0 psi = 227-257 ohms
20 psi = 142-163 ohms
40 psi = 92 - 46 ohms
80 psi = 9-49 ohms
speed
5.3 psi = 20 mph
27.8 psi = 45 mph

Hope this helps. It's not a bad way to learn more about your boat.

surfnturf