The solution depends on what you actually need. There could be different scenarios:
1. just make sure you can get exclusive access
2. force to close/kill application if it is running
3. prevent the usage of application while in process of copying,
etc...

If the #1 above is oK, then this might be enough:
lnfp=fopen("appfile.exe",2)
if lnfp<=0
* file is open somewhere
RETURN
else
=fclose(LNFP)
ENDIF