You will definitely "feel" a drop in performance. Your vapor will drop, and your heat will drop. IF you haven't noticed a drop in performance, pull your battery out after about 3-4 ml and check it. I notice a drop when my batteries get to around 3.7 volts. To ruin a battery, it would need to drop well below that (2.5v) so I don't think you could stand to vape it down that low. I know it seems like a crap shoot, but I'm telling you that you will definitely notice the drop and pretty soon it'll just become instinctual.
Sometimes when I need to change a battery, I'll sub-consciously over-squonk, trying to get my perfect vape back. I won't think about the battery, but I'll "feel" that something is off. Nine times out of ten, changing the battery will "fix" it. When I change the battery, I'll see that it's at 3.6 or so.
Your instincts will develop, and it's noticeable enough that you won't ruin any batteries while you're getting there. Hope this isn't too vague.