There are many ways to validate fields that are created on a dynpro of a dialog screen program. Whether this is to validate a single field of multiple fields at the same time below is a few examples of how this can be coded into a PAI module of the dynpro flow logic.
* Validating a single field on a dynpro screen via a PAI module call FIELD scr_field-ebeln MODULE validate_screen_field "ABAP code for validation contained in PAI module ON INPUT. *Validating multiple field on a dynpro screen via a PAI module call CHAIN. FIELD: scr_field-ebeln, scr_field-ebelp. MODULE validate_screen_fields. "ABAP code for validation contained in PAI module ENDCHAIN. *Validating a dynpro screen field via a direct ABAP table selection FIELD scr_field-ebeln SELECT * FROM ekko WHERE ebeln = scr_field-ebeln INTO ekko WHENEVER NOT FOUND SEND ERRORMESSAGE 001 WITH ' Document Number ' ON INPUT.