Obtain FieldNo from a form control

How can i take the underlying field number of a form control which has a link with a table field ? How can i navigate through all the controls on a form to decide for example in a openform trigger which controls based on fields must be disabled for a user ? I think Navision has no such functionality. Is there a trick or a addon for this ?

This is doneby using the Name of the control, not its ID number. Why notlook at how perissions are handled on the main menu to get an idea of how it works.