IFEXISTS     SYSTEM    NAME|~NAME
    ElseIfExists, Else, EndIf
    AndIfExists, OrIfExists

This command works exactly the same as the "IF" command, but with
different arguments. Look at the if command to understand it.

It lets you see if various things such as PORT, FILE, DEVICE etc.. 
exist in your system, or not.

Example :

TestVar = "MyWindow"
IfExists WINDOW $TestVar
  Say "My Window is open."
Else
  Say "My Window is closed"
EndIf

You can also use a negative form to check if something does 
*not* exist, by puting a ~ character in front of the item
you're looking for :

IfExists PORT ~AREXX
    ; Port AREXX doesn't exist
    run 'RexxMast'
endif