This question is indeed a duplicate of Determine whether browser supports printing. See carpetsmoker's comment on this question and the answer referred to. I agree, that may be the best we can do for now.
Even if window.print isn't part of any standard, it'd be nice if browser vendors implemented and agreed upon some API for detection. Perhaps they're all internally working on an appropriate implementation for the function and so don't want to throw out there something temporary that'll be useless later. Oh, well.