As you have correctly guessed, it will be difficult to monitor your progress (or potential back-sliding) without feedback from partners and instructors. This is why Martial Arts are practiced in group environments (man sharpens upon man). Nevertheless, solo practice is vital, even when you have a regular class that you attend.
My advice for you would be similar in some respects to that I would give to someone not yet able to join a MA class: focus on fitness. Get your cardio and strength training up. Then you'll be in a better position to return to TKD when you are able. Since you have some training already, you can add basic movements to your training, e.g. increasing reps of punches or kicks of each type that you know.
Likewise, you can practice each of your forms. I recommend you devote as much or more time to slow, deliberate forms practice, really hammering on the stances, as you do fast, dynamic forms practice. When form goes bad, it's the stance that is first to go.
As far as making sure you don't go astray, you're in luck since you do TKD. Every ITF form (assuming you're not doing ATA or some other association) is available for viewing on Youtube (in both Shotokan-style and Sine-Wave style), not to mention handy-dandy charts like this one Dan Gun which you can find by doing a Google image search for the form you want.
I advise you to practice your movements in front of a mirror, at least once per practice session. You'll monitor yourself in the mirror for two things:
- Misalignment of your posture (e.g. leaning forward or backward, knees not in the same direction as the toes, fists not properly chambered on hips, chin not tucked, shoulders too high and tense, etc...)
- Extraneous movements that will become obvious tells in sparring. You won't be able to detect the subtleties--you need an instructor for that--but you should be able to see any obvious, unnecessary movements.
Make no mistake, however. You're not going to get much better than you are now practicing alone. My intent was to help you not get much worse. Hopefully you'll only be slightly rusty when you can go back to training.