One way to do this is in a systematic fashion is to look at the course catalog for a university well-known in the field and check the prerequisites of the things you’d like to study.

For example:

> To learn discrete math, I need theory of computation/automata theory.

Not for undergraduate discrete math.

For example, at UC Berkeley, CS70 - “Discrete Mathematics and Probability Theory” - suggests

> Sophomore mathematical maturity, and programming experience equivalent to that gained with a score of 3 or above on the Advanced Placement Computer Science A exam.

https://www2.eecs.berkeley.edu/Courses/CS70/

In this context, sophomore mathematical maturity means having taken college-level Calc I and Calc II. Having taken Intro to Statistics may be recommended.

Getting an AP CS score of 3 or above means the student took Intro to Computer Science in highschool and got a score on the US-wide standardized test for that subject that will let them skip Intro to CS at most US colleges.