One-liner to check whether an iterator yields at least one element?

any won't go beyond the first element if it's True. In case the iterator yields something false-ish you can write any(True for _ in iterator).

In Python 2.6+, if name sentinel is bound to a value which the iterator can't possibly yield,

if next(iterator, sentinel) is sentinel:
    print('iterator was empty')

If you have no idea of what the iterator might possibly yield, make your own sentinel (e.g. at the top of your module) with

sentinel = object()

Otherwise, you could use, in the sentinel role, any value which you "know" (based on application considerations) that the iterator can't possibly yield.