There isn't much that can go wrong with that thing and I suspect many get changed for no reason. It's nothing more than a 4 pole stepper with a total movement of 125 steps. I'm betting your problm lies elsewhere but here's some tech details about the ICS system. On the 7M idle control is based on the following inputs. Some targeted idle speeds are variable while others are fixed.:
1) Engine speed (NE)
2) TPS: ISC is only active when IDL contacts on the TPS are closed. A quick check of the IDL contacts can be done by shorting E1 and T with the engine off and opening the throttle slighty. The code for IDL should be displayed (I forget what it is).
3) Coolant Temp (THW)....for cold idle.
4) Vehicle speed (SPD): While the ECU could simply look at the IDL contacts on the TPS there are obviously cases where the vehicle is moving with the throttle closed. For example, you wouldn't want cold idle active while braking to a stop.
5) Air conditoner clutch (AC): For idle up when the A/C is on.
6) Headlight operation: On the Supra the headlight switch sends a signal to the ELS (electrical load) terminal on the ECU. Other alternator loads are sensed by engine speed. This setup is different from most Toyotas but it works out OK.
7) Battery voltage: The ECU looks at B+ for several reasons anyway.
There are a few other inputs monitored for cars with auto trannies and the inputs mentioned are used together in some cases. For example, for cold idle the ECU looks at coolant temp, engine speed, checks that the IDL contacts on the TPS are closed and vehicle speed is zero. Needless to say engine speed is always being sampled as it's the primary process variable (what needs to be controlled) in the loop. After all, how can idle be controlled if the ECU doesn't know what the engine speed is in the first place?
Other than a winding opening up the only other failure modes are the bearings or gunking of the screw nut or screw itself. ISC valves can be easily tested as per the service manual. As shaeff pointed out, a quick electrical check is to listen for the ISC valve stepping back to full open just after the engine is shut down. That doesn't mean the pintle is actually moving however. Also keep in mind the Supra's ISC system is "smart" in that the values used are periodically re-written in the ECU's memory.
If the ISC valve fails it'll remain at it's last controlled position. This makes sense but needs to be pointed out as it will obviously effect the amount of air bypassing the TB under all conditions. On turbo cars there is a check valve on the manifold side of the ISC valve. It needs to flow air in one direction but stop boost in the other.
Lastly, it's important to remember the ECU uses inputs for multiple tasks. Obviously the coolant temp is used for more than idle speed control. This is why it's painful to read about busted sensors or bad wiring. If you want the car to run right you need to be sure that all sensors and wiring associated with the TCCS are in good condition. Even then a sensor can be out of whack and not throw a code. The MKIII's OBD-I system isn't very bright when it comes to this, it's much dumber than OBD-II.