R – Custom click tracking for adsense

I need to identify my users when an ad is clicked. For example, when user A is online, I need to know that he was the one who clicked the ad unit.

When using my own ads, this was a piece of cake (using URL redirection) – however, now we are planning to switch to Adsense – which renders the Ad via javascript – hence I cannot setup a redirection there.

How do I track – which user which ad unit – in case of adsense?