Knowing the fifth note of each note or at least some of them is useful when forming chords or chord progression. A bit of basics in this post.

When C is the root G is the fifth. How?
1. We can either count from the root through minor 2nd major 2nd etc till we reach the 5th note of the scale – which is the 8th note in the note sequence (C C# …F# G)
2. We can remember what is called the Cycle of Fifths.

The circle of fifths

Since the relation of 5ths appears frequently in harmony and chord construction, the circle of fifths is a basic study material for students of music theory. The circle also has relevance associated with the number of sharps and flats in a scale when writing notation.
the circle of fifths
It visually shows the 5th note relationships when moving clock wise. Fifth of C is G Fifth of G is D etc

The circle of fourths When moving counter clockwise, the relation is one of fourths – F is the perfect 4th of C, Bb is the perfect 4th of F etc.