Look in the VFP help file for SYS(16) and also PROGRAM()

The help for Sys(16) includes this code snippet as an example. It reports out the different levels of calling programs.

STORE 1 TO gnX
DO WHILE LEN(SYS(16,gnX)) != 0
? SYS(16,gnX)
STORE gnX+1 TO gnX
ENDDO