To see all the diff in tracked files but not staged:
git diff
or
git diff path/to/a/given/file
to see the diff only for a file. You can also see the diff in a given sub-directory of your project:
git diff path/to/a/dir/
If you have already staged the changes with git add, you can see what patch you have staged with
git diff --staged
You can also specify a path with --staged.