What is the appropriate method for testing voltage at a connector in the engine bay without stripping the wires?

Use a high impedence volt/ohm meter (5,000 ohms per volt or higher), and back probe at the connector to the terminal you wish to test. Don't strip the insulation from the wire, moisture will enter and corrosion will travel through the wire to the next connector and/or component.

Forgive my ignorance, but how do you 'back probe'

I think Old Master is referring to taping into the wiring at connector or back a ways from it.

But do not cut insulation or poke into wiring with a pin or test probe.
The preferred way to do it is with a special test adapter called a bed of nails.
And always use a high end high impedance volt/ohm meter when testing and computer circuit.

Good parts places or tool dealers should have them.

They have a end with a bunch of fine needles that will probe the wire for test and will not leave a hole in the wire.
If done with real fine needles as the bed of nails test adapter has the small holes in wire will seal back up there self.
MT

Back probe is done at the back of a connector where the wire goes into the connector. The test probe touches the terminal in the connector and doesn't pierce the insulation at all.