Would "joli" go before or after the noun in French? (simple question)

It goes before the noun, for example "il est un joli garçon!"

It would go before the noun.

Explanation:

A way to remember the placement of any adjective in french is to use the acronym B.A.N.G.S.

B→beauty

A→age

N→numbers

G→goodness

S→size

The adjectives in the bangs acronym are the only adjectives that go before the noun.

In your example joli goes in the beauty category, so before the noun.