If my bicycle is wet, it rained.
If my bicycle is dry, it hasn't rained yet.
If my bicycle has ice on the seat, it is going to be a cold ride.
If my bicycle is hot to the touch, I need extra water.
If the trees are swaying, it is windy enough that I shorten my ride.
If the bike is swaying, I need to stop drinking.....

No, really, I check NOAA when I bother. I usually go by whatever worked for me yesterday, and a quick look around when I go outside. And I carry a cheapy poncho, lights, and leg and arm warmers.