back light was switched off don't think it exist.
And i don't know any rapi.dll function to do what you want.
So my suggestion is to trap the time when a user clicks on th pda and compare it. But since you can only trap the event on your application then you must use an API function that will that the events on other forms-application and bring them to you. I think callwindow...something is the solution. Maybe some of the guys here that use API will help.
But the general idea is to trap the event of a form outside your program that te user has interacted with, get them time and when you try to dowload your file, compare the 2 times.