From my experience, one of the most important (while apparently innocent :)) question is:

What (if any) source/version control software are you using?

If they don't use any... keep looking! :)

Another important question is if they use third party components. If yes, which ones. If no, why? The truth is that nobody can develop everything, so, if they still do it in this way, they must have a very solid reason. Otherwise, you may find yourself in a company that dies developing the wheel again and again.

And you should always ask what are the current versions in use for VFP, Windows, etc.

Vlad