How would i know where to apply different scales to a jazz piece?

That's a deep question since the choice of how you apply scales to chords is very personal and will define your own unique harmonic style.

Every master jazz player has their own way of doing it. But to get started, you need to get a good book on chord/scale relations.

Mark Levine: The Jazz Theory Book

(You will spend the rest of your life in there) Report It

Generally, the scales you use are based on chords of the songs.

Listen to the Jazz Greats, and their style, and learn your major scales inside out- a good start. Learn the different modes of those major scales, understand which pentatonics you can use where, which will help you use blues scales. Learn variations on scales such as the bebop scale.
Learn patterns in all keys such as bebop patterns etc.

Oh and get a tutor..... it's really worth it!

This is gonna sound silly.

Learn the scales - inside and out. They are good for you.

Then forget them.

Play notes. There are no 'wrong' notes. If you hit what YOU think to be a 'wrong' note - play it again.