How to know you have tailwind?
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.