The most recent version of this documentation can be found at https://docs.dyalog.com/20.0/.
The )METHODS system command lists the Methods that apply to the object associated with the current space.
For example:
⎕CS 'F' ⎕WC 'Form' )METHODS Animate ChooseFont Detach GetFocus GetTextSize Wait
)METHODS produces no output when executed in a pure (non-GUI) namespace, for example:
⎕CS 'X' ⎕NS '' )METHODS