The read is whether he's doing a poke or another normal. That's kinda the only reason Kitana is able to open up Johnny Cage since if you can just react to every poke, you'll just sit there and block whatever she does, and if she pokes, you'll just counterpoke and start your pressure. Why else would Cage get hit with standing 2 in certain situations? She would almost literally not be able to open him up ever otherwise.
I don't think it's a read in the sense that it's a total guess. But once you understand this game and its neutral, you've gotten conditioned to understanding the poking game and when players like to poke, so in the footsie game, when both players are say, at sweep distance, someone will be looking to poke to get his offense going. Recognizing this, you're waiting for him to poke (so you're expecting it, which makes it not a total reaction), and you just let go of block and counter-poke. So I wouldn't say it's 100% read as in a complete guess otherwise players wouldn't be counterpoking so well and so often, but it is definitely not pure reaction as in you clearly see a poke coming, block, and counter it. You react to the movement, which you've been conditioned to believe is going to be a poke, and most times, it is.