From Forum Nokia Wiki

#include <w32std.h>
Link against: ws32.lib

CWsScreenDevice& iScreen = *(iCoeEnv->ScreenDevice());
TSize originalres = iScreen.SizeInPixels();
 
TSize resolution1(320,240);
 
if( originalres == resolution1 )
{
 // It means the resolution is 320 x 240
 // Do something.
}