Run glxgears, which will calculate how many frames per second it can display. On my PC, an 850 MHz Pentium III, it displays around 150 FPS with direct rendering turned off, and over 700 FPS with direct rendering turned on. Your mileage may vary.
For confirmation, run glxinfo. If direct rendering is enabled, the output should include the line:
direct rendering: Yes
Lastly, check the X log file, which will probably be XFree86.0.log. This should include a line similar to
(II) R128(0): Direct rendering enabled
As I understand it, if the X log shows direct rendering to be enabled, but the GLX tools say it isn't, then the problem lies somewhere in the GL libraries. But I could be wrong.
I have also observed that installing new versions of kdebase and kdelibs from the RPM will stop DRI from running. If you have just reinstalled kdebase and kdelibs, you should reinstall XFree86 (see below) and reboot.