We'll only need a couple of tools for this - pedal/module and power supply, a Multimeter, and a couple of crocodile clips.
Connecting the Multimeter
We're going to use the Multimeter to measure the current draw (in Amps). To measure current, we have to insert the Multimeter in series with the device we want to measure:
For a pedal or circuit which is normally powered from a single power supply or battery, we want to insert our Multimeter between the positive (9V) supply and the power connection of the circuit (PCB).
The power supply ground (GND) or negative of the battery should connect to circuit ground (PCB). Set the Multimeter to the mA range, and the current measurement should display on the meter:
Synth Module (Dual Power Supply)
For a synth module which is typically powered from a dual power supply (+12/15V and -12/15V), we will want to measure the current on both the +12V and -12V lines. To do this, measure the current on either the +ve or -ve rail while the other voltage rail and GND connections are in place, then repeat for the other rail.
This LFO module below is drawing 15.52mA on the +12v power line:
You can also use this method to measure the current draw of any electronic device (not just pedals and synths). What are we actually measuring when we measure current? The numbers on the Multimeter are the rate at which electric charge passes a point in a conductor over time.