If you can find a modeline what your output is currently using you can use online services to check framerates it provides.
"xvidtune -show" prints the current modeline, but the refresh rate calculated from it will not be the same as the actual rate.
My used modeline is (1080p60) $ xvidtune -show -display :0.0 "1920x1080" 148.50 1920 2008 2052 2200 1080 1084 1089 1125 +hsync +vsync
Which compiles to 60Hz output on theory level at epanorama calculator. If it is 60.001Hz because of clock Inaccuracies the TV will synch to it, no problem. As I use my VDR's X-output only for TV-based web browsing with randon youtube/vimeo.
However I am now at office, so I cannot use "get-edid" to read all modelines TV is announcing for PAL-modes/timings. And how Nvidia is altering the proposed timigs as horizontal synchs must be at multitude of 8 (so you cannot have f.ex resolution 1924x1080).
- Jori