The fuel pump is powered through the fuel pump relay pin 87.
On my 86 300E the relay is behind the battery and a black flap. If you remove the relay and measure pin 3 from left (viewing from front of car) on the first row - compared to ground, you would get the fuel pump resistance. (not sure if you'd get anything reasonable though)
Here's a picture of the different relays found behind the battery with their bottoms up...
Klima relay (top left)
Fuel Pump relay (bottom left)
and the 'lil one to the right is the OVP (overvoltage protection relay.
This is how they're found behind the battery too.
If you want to measure amperage you'd want to series the instrument with the fuel pump and pin 87 on the relay...which will be a bit tricky...
99 W210 E220 cdi sedan automatic (248.000 km) 210.006