Exemple :

if not exists c:\test.txt
=> ?? to have an alert in the console
else
=> apply a package
endif

if I do nothing, the console message is "installed", even if the file is missing. I would like to know, in the console, the result of my test in the script.

http://www.newboundary.com/support/docs/Tips_and_Tricks/centralerrorlog.htm

An example script could be:

if exists c:\test.txt
/InstallPackage \\server\share\package.pwc
else
/set error "test.txt does not exist"
/IP \\server\share\error.pwc
endif

error.pwc would then add an entry to a central INI file that resolves both a %computername% and the %error% variables. Then you could check the INI file and it will tell you which computers didn't have the test.txt file.