To understand this topic, you should have a basic idea about Functional Dependency & Candidate keys and Normal forms .

Steps to find the highest normal form of relation:

Find all possible candidate keys of the relation.
Divide all attributes into two categories: prime attributes and non-prime attributes.
Check for 1st normal form then 2nd and so on. If it fails to satisfy the nth normal form condition, the highest normal form will be n-1.

How to find the highest normal form of a relation  : https://www.geeksforgeeks.org/how-to-find-the-highest-normal-form-of-a-relation/