Hi Fabian
One way could be to open the exe file you want to check using low level file function fopen() and then search for the string "VisualFoxProRuntime". If it is a VFP exe then this string will be present. This string will be within the first 6kb of a VFP app and within 13kb of a VFP5 app (or at least thats what I find with my apps).
In the End, we will remember not the words of our enemies, but the silence of our friends - Martin Luther King, Jr.