2007 Mercury Montego V6, Need to test the alternator output independent of the wiring harness and computer

We’ve had the alternator rebuilt once and replaced it entirely with a brand new one. We’ve replaced the battery twice. Mechanics keep insisting it’s the alternator. And I quote "it’s never the wiring harness". I’m not a mechanic. I’m not an auto expert. But I’m handy with a wrench and have been up to my arm-pits in an engine’s guts (auto and marine) more than once in my life. And I fix computers for a living… so diagnostics is literally my bread and butter. So when a mechanic tells me it’s the alternator after rebuilding the original alternator and then replacing it with a new one… my BS meter pegs.

So I need to test this for myself. I need to eliminate the alternator as a possible cause. Alternator on a 2007 Mercury Montego should output 14 volts (correct?). I’ve seen the computer output. The computer says its outputting ~14 volts when the car has just been cranked but drops to ~12.5 volts after the car is actually driven around for a short distance.

How would I go about testing this so I’m testing only the output of the alternator… no possible interference from faults in the harness or misbehaving ECM? If I need to get out a multi-meter, I’ve got two handy.