How can I see what has changed in a file before committing to git?

You’re looking for

git diff --staged

Depending on your exact situation, there are three useful ways to use git diff:

1. Show differences between index and working tree; that is, changes you haven’t staged to commit:

git diff [filename]

2. Show differences between current commit and index; that is, what you’re about to commit (--staged does exactly the same thing, use what you like):

git diff --cached [filename]

3. Show differences between current commit and working tree:

git diff HEAD [filename]

git diff works recursively on directories, and if no paths are given, it shows all changes.