I'm merging in a remote branch that may have a lot of conflicts. How can I tell if it will have conflicts or not?

I don't see anything like a --dry-run on git-merge.

As noted previously, pass in the --no-commit flag, but to avoid a fast-forward commit, also pass in --no-ff, like so:

$ git merge --no-commit --no-ff $BRANCH

To examine the staged changes:

$ git diff --cached

And you can undo the merge, even if it is a fast-forward merge:

$ git merge --abort