K N:
Daniel said in MCTB that "you'll know when you have it", so if I have to ask, I guess I don't, but I'll ask just in case anyway. ;)
Lately I've mostly practiced (or at least tried to practice) noting, but I did a couple of experiments with breath counting to gauge my concentration. I found that I could at least count (exhalations) to ten without losing count for ten minutes. Occasionally there would naturally be some thoughts, mostly during the inhalations, but they weren't prominent enough to disrupt the counting. Would this indicate access concentration, or is there more to it?
Generally speaking, if one is able to maintain focus and concentration on the meditation object for 3-5 minutes without distraction or without unnoticed distraction, then one has reached enough concentration (meaning neighborhood or access concentration) to begin pursuit of absorption concentration (or
appana samadhi, which is "fixed" concentration).
If you should reach
appana samadhi, you will know it by the qualitative difference it presents. The mind is able to remain effortlessly fixed on the meditation object,
undisturbed by any exterior or interior distraction phenomenon. When you are able to reach this kind of
samadhi, it makes contemplation on a subject (like the five aggregates, or noting the three characteristics of phenomena) equally effortless while bringing to the surface insight which was before perhaps overlooked. Absorption concentration can be a very pleasant experience, presenting the impression that one could sit "forever" in this state of contemplation. It is qualitatively a superior state, wherein the mind is unequivocally established on the object, pliant, workable, having gained imperturbability. There should be no mistaking it for anything else.
When samatha is the goal of pursuit and absorption is attained, the experience will be one of a deeper and deeper experience of calmness and tranquility. When vipassana is the goal of pursuit during absorption, the experience will be of contemplation and insight gathered about the object of contemplation. The mind will have a tendency to be a bit more active in vipassana jhana than in samatha jhana. Otherwise, the experience of the depth of concentration is quite similar.
K N:
I've noticed quite often lately that after a few minutes of sitting, once my mind settles down, there's a shift in perception where the darkness of my closed eyelids seems to expand into a more three-dimensional space. In everyday life I guess my awareness of my body mostly resides somewhere around my eyes, and once this shift kicks in, it's as if this awareness expands. At the same time, my body starts to feel somehow less solid, or perhaps just more in the background. Does this have something to do with access concentration, or is it perhaps a sign of approaching the first jhana, or something completely different?
It's kind of difficult to say from your description. It might be an indication of this or that; and then again, it may not. If this "three-dimensional space" is a pleasant space associated with the breath, then it may indicate a deepening of the concentration level. If not, then it's just a perceptual anomaly, meaning perceptual fluff that is best let go of.