Instruments and glide calculators help, then pure skill
Do a search on speeds to fly theory... it can all get a bit complicated and then it is also based on what you are attempting... speed over a course - average lift - sink etc.
Basically you learn with experience - best glide speeds - best LD are the things you need to know. At its most basic level if it moves up in your field of view you are not going to make it, if it moves down you should be right, if it stays in place it depends on the net lift sink you get on the way.
Yep Speed/energy = altitude, but it is finite of course and knowing what you have in the bank is the skill.
The height you can gain is a factor of the length of the cable and whether it is reflex (run around pulleys) or static (pure tow), you need a pretty long cable for decent altitude. Most gliders can do aerobatics as the have a very broad speed range and retain energy better than most other aircraft...