How to autodetect screen resolution

Question

i'm using default ati driver, because X always crash when i use fglrx and fglrx-updates driver

when i use randr, the output was:

Screen 0: minimum 640 x 400, current 1280 x 1024, maximum 1280 x 1024
default connected 1280x1024+0+0 0mm x 0mm
   1280x1024       0.0* 
   1152x864        0.0  
   1024x768        0.0  
   800x600         0.0  
   640x480         0.0  
   720x400         0.0  

but i know that my monitor is wide screen, i just forget what is the maximum resolution 1920x???

so, how to autodetect my monitor resolution since xrandr failed to do this..