Great advice from Zamataki. From another perspective, assuming you want to enter the corner at a specific point, if the car understeers and won't turn into the corner, then you are going too fast. If the car spins out or goes onto the grass on the inside of the corner, then you entered the corner too slow. Of course, all of this assumes you are using proper technique to start with. It just takes practice is all. Eventually you should be able to "feel" the entrance speed, rather than know it. For example, I can't look at a corner, even if it's one that I drift regularly, and say "you enter this corner at X speed." I don't concern myself with such details, to memorize the entrance speed of each and every corner would be
Also, I noticed in real life that you are entering the corner at almost exactly the same speed as if you were gripping. Unfortunately in GT3 you can't really feel the point where the tires begin to lose grip. Anyway, I enter the corner as if I was going to grip it, but I brake just a little bit early to give myself time to load the suspension, and a little hard to slow myself down just below optimum grip speeds. The more extreme your angle is, the slower your speed will need to be. Since your steering wheel can only countersteer so much, there is a certain minimum speed you must enter the corner at, otherwise you will not have enough speed to follow through at the exit.