How to test a merge without actually merging first

I don't think there is a way of simulating what will happen until you try the merge. However, if you make sure that the output of git status is empty before you do the merge, it is quite safe to just go ahead and try it. If you get conflicts, you can immediately get back to the state you were at before with:

git reset --merge

Since git 1.7.4, you can also abort the merge by doing:

git merge --abort

(As the commit message that added that option explains, this was added for consistency with git rebase --abort and so on.)

You can use git merge --no-commit to prevent the merge from actually being committed, and if you don't like how the merge works out, just reset to the original head.

If you definitely don't want to finalize the merge, even if it's a fast-forward (and thus has no conflicts, by definition), you could add --no-ff as well.