I gots a question about the lane phase :l

I'm a mid main currently trying to climb my way back to platinum. I have a question about the lane phase: **How do you know when your opponent will hit 6 before you? **

I'm aware that if they've roamed or conceded a kill to me, I'll have the EXP advantage, but If the lane is even all the way til level 6 (with near perfect cs), how do you know who will hit 6 first? I can easily time my level 2 and level 3 window, but i loose count of waves eventually and can't determine if I should lock the lane around my turret and do an all-in at 6, or play it back before my enemy hits 6.

help pls :c