CHECK(ER) and Field Validation

How can I let my program know that CHECK(ER) is being used and to run a
validation routine on the field that has CHECK(ER).  I'm using CHECK(ER) on
a field I want validated as soon as the user enters in the last position of
the field.  Is there a way to have a validation routine run only when
CHECK(ER) is being used.  The program is an RPGIII program.  I'm open to
easier/alternative suggestions of accomplishing this same task.

I would like something similar to:

EXFMT DISPLAY

*Validate XFIELD because there is a display attribute of CHECK(ER)
   on the field and the user has entered data into the field
================================

================================

*NORMAL PROCESSING
=============================


=============================

If there are other ideas on how I can accomplish this, please let me know.

Thanks,

Frank