How do I know if a file is complete with being written to the disk?

I have a program and it uses IF File(XYZ)
And if that file is there my program will use that file.

On LARGE files the IF FILE() will return .T. even though the other program is still writing that file.

So I get an error.

I did work around by putting a Inkey(5) to delay using the file after it appears, that worked fine.
However I do not want to wait X seconds every time and what if it was a very large file and x seconds was not enough??