If you follow the 3 to 1 technique that Paul teaches in the Photoreading Course you'll be in the accelerated learning state. Intially it might not be as "deep" but thats the idea of training your mind.
When you keep repeating the process you are committing to telling your mind what you want it to do ... it automatically find the best level for you.
The clues that you are relaxed enough are any 1 or more of the following
1. You did the relaxation exercise/stated the affirmations
2. You can see the blip page... not everyone see it.
3. you notice your breathing has changed ...deeper more even.
4. you feel calmer a sense of quiet about your body
5. You feel more focused than you might normally do
6. You might notice tingling in your body.
7. Any other subtle sign of change in the way you feel.
Being in the right "optimal learning" state changes day to day too. Mostly it what is right for you and if you follow the relaxtion steps and trust the process - you're there.
It really is a case of practice makes perfect. Also you can be so used to being in the optimal state you don't notice any difference.
Closing your eyes for a moment mentally giving yourself instructions and affirmations with your eyes closed then trusting the process is all it really takes. When you do that you are entering the optimal learning state. As you said it's subjective, how we feel, what's happening around us, how hot or cold we are all make your relaxation feel different. When we train our mind by repetition we can reliably reach that state without feeling "relaxed" when we need it.
Hope this helps