I think that the best choise is to measure the voltaje drop in a resistor.
You should put a resistor (a smaller one, like 1 ohm) in serie whith the current source. Then, you should use a operational amplifier to measure de voltaje drop across the resistor.
You will have a lineal relation between the current and the voltaje (PE whith a 1 ohm resistor, 1 mV is 1 mA across the resistor).
I hope that it helps you.
Excuse my English.
Added after 3 hours 49 minutes:
Hello again.
I want to explain you by a circuit, that it's better than words.
This circuit has a gain of 1 (1 mA on R results in 1 mV on Vo).
If you want another gain, here is the formula.
There are current ADC, but it's the cheaper way, and it's very simple.
I hope that it helps you.
Excuse my English.