Turning in too LATE can also be indicated by your apex - it may be too late, in which case you end up on the inside of the track at the exit so you are not using all the track (which means you didn't straighten out the corner as much as possible, and thus your entry/corner speed was lower than it could have been), OR something else commonly seen is turning in too late and not being able to GET to the apex you wanted.
For example, at my track day the other day I watched riders take a a very late turn point (they were watching each other, I think), and then never get anywhere near the inside edge of the track at the apex; I'd say they were at least fifteen feet wide of an apex cone that had been set at the edge of the track by the trackday provider. Ergo, they didn't really straighten out the corner as much as possible and then they had to compensate for that by either using a very low entry speed, or by stalling on the throttle to keep from running wide.
RChase's statement above is a nice restatement of the definition of a good turn point: one that allows you to make one steering input, and apply good throttle control through the corner.
Sometimes when you use an entry point that is too late, you make your turn angle into the corner much tighter (more acute) than it needed to be; anybody remember which Level 3 classroom topic and track drill addresses that?
IIRC, that is the Attack Angles drill, which I really learned a lot from. One of the many things I like about CSS is that we are actually encouraged (albeit mainly during this drill) to use different lines / different attack angles and get comfortable with different approaches to a corner. As a result, there is a much better understanding of what needs to be done differently as you enter, apex, and exit the corner. It really helped with my passing ability. Most track day organizations want you to always "hold the race line", pretty much to a fault.
There is one turn at NJMP Thunderbolt that, because of the way it is configured, a late apex actually slows you down. I had been consistently turning in too late, hitting an apex that was far into the turn, and not getting a strong drive out (and not using all the track available to me). Finally during a break, I went over to watch this corner and noticed how the coaches & faster riders were turning and apexing much earlier than I had been. They were getting great drives out as well. So I started to experiment with a few different turn in points and found my sweet spot. What a difference.