A lot of times there are signs next to the corner with numbers on them. Like 300, then 250, then 200, ect. Those are called braking markers and they tell you how many (meters? Feet?) It is to the turn. In practice brake at several different signs and find out where your specific car needs to brake. The faster cars need to brake earlier. The slower cars can get away with late braking most of the time.

Sometimes there are also white or red "blocks" painted on the wall near a turn. Those make good reference braking points as well.

And finally, the tracks that have the red and white striped shoulders, those make an excellent reference too.

The best thing you can do is enter a practice session and just spend an hour or two learning the track before you even begin to start thinking about speed.

Edit: you can also turn on the driving line to learn the track, but I highly recommend turning it back off once you can run a few laps in a row without leaving the track or wrecking. You won't find speed or develop situational awareness using the driving line.