How to check which graphics card is driving my display?

The system is setup with one of those hybrid Intel/ATI offerings (muxless). After a bit of fiddling with kernel settings and drivers I got both cards working, I think (adding radeon.dpm=1 in kernel settings and using only the opensource drivers). However I can't figure out which card the system is using. From my understanding with the newer kernel (3.12) amd dynamic power management will power down/ power up the card when needed, so in theory it should be using the integrated hd4000 for most of the time, however I can't find an easy way to check which one is in use. lspci | grep VGA only lists all the cards, it doesn't specify which one is currently in use. Some steering in the right direction much appreciated. System Debian 7 stable, 3.12 amd64 Kernel 7670M AMD + Intel HD4000