Is there an way to make sure the screen data has been populated (i.e. is not all black) before running an operation?

Hello,

I'm trying to write a screenshot script over VNC, and I find myself needing to include a sleep() statement after I connect to the screen. Without the sleep statement, the screen is black in the .png. Is there a way for me to wait until the screen has initialized in a more intelligent manner? Thanks!

Niko