A common problem when prescribing a shoulder rehab program is that once the athlete or patient has acheived the goals of normalising their shoulder strength, it is often difficult to either monitor the strength after being discharged, or know when the strength declines enough to cause problems again. A simple way is to correlate the amount of reps and sets before fatigue during your exercises and ensure you can match this in the future. Eg if your shoulder was given the all clear, and you were able to do 3 x 30 reps of the exercises, then it would be quite safe to assume that if you can maintain this routine, then you have maintained your strength.
However one easy and inexpensive way is to use a set of luggage scales and actually measure your own strength. Take the example of the external rotation exercise, one would place the scales in a similar position as where the resistance band is and slowly pull. If you know what your max strength is, you should be aiming to achieve this over time.
For further questions on this, please ask the physios here at OrthomedPhysio...