Allen's hummers look very similar to our common Rufous hummers, but their range has been well south of us. Still, with many expansions happening to the north, wouldn't be too surprising if they started showing up here.
The easiest way to tell them apart is to look close at the adult female back and head: Rufous is pretty much all reddish , whereas Allen's has green on back and top of head. The tail feathers are different, but hard to catch sight of. The males' breeding flight patterns are quite different, so that could be another way to tell them apart.