Amperage test, not voltage test.You are correct that doing a voltage check from the ground cable to the negative battery terminal would work, and that's the better method.
That can work. A bright bulb indicates substantial current draw, a dim bulb indicates reduced current draw. No light at all means either no current flow...or the bulb is burned-out.When checking for parasitic draw, I strongly prefer using a test light with an incandescent bulb.
Most test lights sold now have LEDs instead of incandescent bulbs, so this wouldn't apply to them.
It would be best to "calibrate your eyes" by using the test light, and then confirming with a meter until you get a feel for brightness vs. amperage on whichever test light you're using. Different bulbs require different amperage for full brightness.