How to determine maximum resolution

I have a Thinkpad T23 that comes with a 14 inch screen with a
resolution of 1400 x 1050. When I select DISPLAY PROPERTIES -->
SETTING ---> ADVANCED --> ADAPTER --> LIST ALL MODES, the maximum
resolution is indeed 1400 x 1050. Next I attach a 1280 x 1024
external monitor to the laptop and this time the highest mode
supported is 1280 x 1024. The 1400 x 1050 mode dosn't show up any
more. Then I attach a 1600 x 1200 monitor to the laptop and this time
it shows 1600 x 1200 is highest resolution supported. It appears the
supported modes are based on the monitor used. Now, how do I find out
the maximum resolution of the video adapter in my laptop? Thanks.

Michael