From this page of the Baldurs Gate 3 guide, you will find out what companion approval is and learn how to check whether the current party member is favorable towards you.
What is approval?
Each member of your team has their own character and is guided by different rules. When you are performing story tasks, you make different decisions that they may or may not agree with. Their reaction can be viewed through a message appearing in the top left corner of the screen.
The higher the approval rate, the more sympathetic the character is to you. However, you must be aware that because of the diversity of your companions, differences are sure to appear. One action may significantly improve your relationship with a particular character at the same time lead to a dislike from someone else.
Where to check the approval rating?
You can check the approval indicator in the "character sheet" tab. It contains the most important information about your team. You will find out from there, among other things, what their statistics are, what class they belong to and what their attitude is to you.
This parameter has no numerical indicator. The character's attitude towards you is determined only verbally.