How to know you have tailwind? I think I missed a lot of opportunities to go faster when cruising and not taking advantage of tailwind. I find that if I put short effort in acceleration, the tailwind will take care most of the speed maintaining work.

For big tailwind I can usually see leaves rolling on along my general direction. Lately, I think if wind noise is more quite, I might be in tailwind?

So how can you tell if you've got tailwind or wind direction in general? (without flag anywhere nearby)
The most accurate way to do it would be with a pitot tube to measure wind velocity and then to contrast that against speed-over-ground from your GPS or wheel speed sensors. 

Before one laughs about the science-fiction nature of this, there's a product that does so: The PowerPod (link is to DC RainMaker's review of it). The Isaac software that comes with the PowerPod allows you to extract effective wind speed and direction amongst other things.