Hi,
I am sure this has been done before but I have just knocked up a little Windows app that you can use to time your motion detection response.
It works by displaying a black rectangle that you point your camera at. When you press the app button the rectangle changes to white with a millisecond timer over it. The time displayed on your snap will be the md response time. You can adjust the timer run time if you want to time subsequent exposures. Use a fast shutter speed.
It should be accurate to -0 / + screen refresh time. For instance, a 60 Hz screen should give results to -0 / +17ms accuracy.
Download and unzip the attached file and then run the mdTimer.exe file. Please let me know of any problems or enhancements.
enjoy
Davo