Checking fields in TV Dialog

I am writing a program for data entry using Turbo Vision.  In one input
field I would like the ability to use a "?" as a flag to call another
routine.  Is there any way to check the input field before going to the
next field in the dialog box? ( I want to use the "?" to call up a search
routine when entered instead of a name string ).  I know that I could read
it's value after closing the dialog, but I need to be able to check the
contents of the input field and act upon it before moving to the next
input field.