IsLabel()

Returns a non-zero number if the specified label exists in the script.

TrueOrFalse := IsLabel(LabelName)

Parameters

LabelName: The name of a subroutine, hotkey, or hotstring (do not include the trailing colon(s) in LabelName).

This function returns a non-zero number if LabelName exists in the script.

This function is useful to avoid runtime errors when specifying a dynamic label in commands such as Gosub, Hotkey, Menu, and Gui.

Reports "Subroutine exists" because the subroutine does exist.

if IsLabel("Label")
    MsgBox, Subroutine exists
else
    MsgBox, Subroutine doesn't exist

Label:
return

Reports "Hotkey exists" because the hotkey does exist.

if IsLabel("^#h")
    MsgBox, Hotkey exists
else
    MsgBox, Hotkey doesn't exist

^#h::return

Reports "Hotstring exists" because the hotstring does exist.

if IsLabel("::btw")
    MsgBox, Hotstring exists
else
    MsgBox, Hotstring doesn't exist

::btw::by the way