When a circuit has a current output, there are two equally important specifications for it:
- The current range (minimum and maximum current),
- The compliance voltage range (minimum and maximum voltage).
You cannot expect to use a current output if both of those two numbers are not available. So the first step will be to know the compliance voltage range for your current source.
Recall that a current source has infinite impedance. So, there needs to be a voltage source (a resistor - could be 0 ohms i.e. a short, a battery, etc.) that determines the voltage. As long as the voltage is within the compliance range, the output will act like a current source, and will be able to provide currents within the specified range.
If the compliance voltage range is down to 0 volts or close to it (say <0.1 V), then you can connect a current meter straight across the output, and measure the current directly.
If the lower end of the compliance voltage range is higher, e.g. 1-5V, then a resistor won't help, since there's 0V drop across it when the output current is 0mA. Instead, you'll need a voltage source.
A lab power supply connected in series between the current source and the current meter will work fine for this purpose, but even simpler: a battery will work too.
If the compliance voltage is, say, 1V-5V, then an AA battery will do fine as a voltage source. It will be recharging and thus may fail prematurely - but for small currents this won't be a problem if the tests don't take long.
If you want to be testing such an output for a longer time (a day), then a discharged AA Ni-Cd or Ni-MH cell will be perfect: the voltage drop while charging will be above 1V, and the cell can easily pass the 100mA current, and a 1000mAh capacity cell will happily absorb 100mA for 10 hours.