If variables don't exist, their value as string is generally a "scalar bool array string 0xe0ffffef" error.
Compatibility:
Version ArmA 1.00
For OFP version, see scalar bool array string 0xfcffffef.
Examples:
hint format ["%1", undefined_variable]
hint (if (format ["%1", undefined_variable] == "scalar bool array string 0xe0ffffef") then [{"Variable undefined"},{"Variable already defined"}])