Graphics mode change should be instant. If it's not, then it is the monitor's fault. Being the monitor's fault, there may be no way to detect when it's finally begun displaying the image.
But if there is any way, the only way I can think of might be to check the scanline that the monitor is currently displaying. It might be worthwhile to write a program to print the current scanline it detects to a logfile both before and after a graphics mode change and see if it gets stuck at 0 for a little while. If that is the case then maybe you could wait until it's not 0 to begin displaying graphics.
I'm highly doubtful that this will work, but it's the only thing I can think of that might work if the monitor is at fault.
But if there is any way, the only way I can think of might be to check the scanline that the monitor is currently displaying. It might be worthwhile to write a program to print the current scanline it detects to a logfile both before and after a graphics mode change and see if it gets stuck at 0 for a little while. If that is the case then maybe you could wait until it's not 0 to begin displaying graphics.
I'm highly doubtful that this will work, but it's the only thing I can think of that might work if the monitor is at fault.