comparator - How to tell when an LM334 is limiting the current?

I have an LM334 based constant current source configured to supply a max of 1 mA.

The load has a variable impedance. My goal is to make an alarm of sorts when that impedance exceeds a threshold, and the way I want to do that is to detect when the 334 is in current limiting mode.

When it's passing less than 1 mA of current, I would expect it to be acting like a very low-value resistor, meaning that the voltage drop across it should be low. When the 1 mA of current is reached, it should begin to increase its resistance, thus increasing the voltage drop across it.

I'd like to measure the voltage drop and light an LED (really an optoisolator) when it exceeds a threshold.

At least, that's my thinking at the present. If anyone else has an idea of how to light an LED when an LM334 is actually limiting the current through it, that would work too.

^{simulate this circuit – Schematic created using CircuitLab}

In the schematic, I1 represents the LM334. I don't see a way in circuitlab to represent an actual LM334, so the simulation likely won't operate correctly. R1 represents the ground impedance - the goal is to detect when it's value exceeds 10k. At the moment, I'm thinking I'd like to find a way to light an LED when Vin-Vout > 1V. From there, it's just a matter of tuning either the set resistor of the LM334 or the voltage threshold to trim to the setpoint.

Note as well that the actual circuit ground is on the top side of R1 - Vout is the ground potential for the purpose of this circuit. The actual earth is on the far side of R1.