The Moon sign gives an insight into the deeper psychological ocean churning inside of a person and it can be used to find mental compatibility between 2 people.
The underlying principle is: Find out the signs in which the Moon of one person has maximum bindus (6, 7 or 8) in the Bhinna Ashtaka of the Moon. The other person should have one of these signs as his/ her lagna or Moon rashi, for good mental compatibility.

A similar consideration with Venus as the focal planet will give an idea about the sexual compatibility between two individuals. Mercury may be looked at likewise for intellectual compatibility.

Use the Cosmic Insights app to check the Bhinna Ashtakavarga of planets in your birth chart!

Found this technique from Vinay Aditya quite useful!