If you feel intimidated around other men regularly, you're not an alpha. If you meet other men and not only can deal with that, but you can easily get along with them without feeling the need to prove yourself or to measure dicks, you're most likely an Alpha.
The difference in dynamics with women between A and B's is a byproduct of them inherently sensing this, not the direct way of judging it.
It's about if you can handle yourself, not (only) physically, but socially, in times of difficulty and also that when you're under social/life pressure that you don't shit the bed.
There's probably some correlation with brainwaves and I get what you mean, but the way people use the terms alpha and beta in this sense is relating to social dominance and really it boils down to your sense of self-worth and coping skills.