You measure the voltage across the fuse. Then you look up the fuse's resistance in a datasheet. Current is Voltage / Resistance.

AFAIK the fuses have contact points you can use to measure the current.

You can check continuity across the contact points, but measuring a 75mA current via the voltage drop across a 0.0034ohm resistance (give or take) means reading 0.2mV, which is… borderline for a consumer multimeter.

I wonder if one couldn't identify the time it takes for the battery to get to some low point (50%?) and then determine the order of magnitude draw they would be looking for. 75mA would take a long time (I think) to drain the battery to where starter can't turn over (like a week?).

A multimeter is cheap and really easy to use. This is the safest way also