How do I see what I am about to push?

Pushing shows a preview of the files involved before it sends anything, in the Diff Review window. That is the direct answer, and it is one button.

If you want the commits rather than the files, the push button counts them: the badge on it is how many commits the remote does not have. Clicking the branch’s remote in the sidebar scopes the history to it, so you can see exactly where the two diverge.

For the whole difference as one diff, compare your branch’s tip with the remote branch’s tip.