In the most recent beta patch, Blizzard just re-added a way for players to monitor their frame rate. This feature had previously been in the game, but was removed a few beta builds ago.
To check your frame rate in-game, hit CTRL+R on a PC, or CMD+R on a Mac.
Keep in mind that frame rates in the beta are not as good as they will be once the game goes live. Players should expect to see a marked increase in overall smoothness of play once the game is released.
It’s worth noting that the most recent beta patch has plagued Mac users with a drop in frame rate. Patch 16 seems to have cut my FPS to about 1/2 of what it was a few weeks ago. Again, this is only for OSX users, as far as I know.
Playing the game on my 3 year old 3Ghz Core 2 Duo 24″ iMac w/ a GeForce GT 130 graphics card, I see an average of 20fps on max settings. If I turn everything down to low quality, I see an increase of about 8fps. Both of these numbers will be noticeably higher in about a month – when Diablo 3 is released.