Besides the date on the calendar, I mean.
There are a couple of sure signs:
-The leaves are turning (OK, that's another obvious one)
-I had to close all the windows in the house
-I'm sleeping in socks
-There's an extra blanket on my side of the bed
-There are Halloween decorations and Christmas decorations on display simultaneously in Costco
And. . .there is snow on the New York mountain range once again.