... is by using this site:
http://www.flightradar24.com/
Click on any airplane and it'll give you data on its speed, altitude etc. But it can be used to estimate wind conditions aloft. Now, the average cruising speed of a jet airliner is around 850-900km/h. You can click on aircraft travelling roughly east or west to estimate the presence of a jetstream. If the aircraft at cruising level travelling east are doing a great rate of speed (950km/h+ , sometimes even well over 1000km/h caused by a strong tailwind), and those travelling west going relatively slowly, e.g. around 700-750km/h, conversely they are being affected by headwinds, indicating a significant jetstream in the area. If you find both east and west bound flights are doing a similar rate of speed, then conditions aloft are generally settled.