Most reliable method to detect a merge commit in pre-commit hook

Hi,

I'd like our pre-commit hook to skip some checks if the commit that's 
about to be made is a merge commit. What's the most reliable way to 
detect a merge commit from within a pre-commit hook script? Currently, 
the only way I see is to check whether .git/MERGE_MSG exists, but I'd 
feel safer to check whether the commit is going to have more than one 
parent or so. Is that possible / a better detection?

-- 
Sebastian Schuberth