Put a voltage divider between V+ and the signal input.
(trimmers are frequently used so the "scale" can be calibrated)
IIRC there was a schematic posted somewhere in this thread.
Edit: check page 30. ;)
A vacant U2.... sounds like a good time for a sanity check.
Measure the resistance between the output pad (the pad closest to the front of the PCB) of U2 and ground.
What's the reading?
(should be in the high K ohm range and will wiggle around as the caps do their thing)
To this---------> And the datasheet for the Tantalum Capacitors installed in the C1 positions (http://www.mouser.com/ds/2/212/F3100_TaDipRad-507115.pdf) Screen cap----------> I'm wondering if they are still installed backwards. The idle current for a Mini3 should be 24mA.