Run a wire or two from battery to passenger area. Hook up a voltmeter and observe voltage while driving at various engine and road speeds.

There's a convenient ground on the tab at rear of auto-trans shift lever housing. I often use an aligator-clip test lead on that one. Or you can just string a double wire from the engine bay.

You could get 12V battery power for your voltmeter from the fuse panel but you might not want to drive around with loose hot wires near your ankles, clipped to equally hot terminals on the fuse panel.
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Sven: '89 245 NA, 951 ECU, expanded air dam to 7' from ground, forward belly pan reaches oem belly pan, open-front airbox, E-fan, 205/65-15's at 44 psi, IPD sways, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245