Reporting Form Errors

Sometimes I pass values from a form to its associated
action by assigning that value to hidden field in the form.
The intention is to use the passed value to modify the
behavior of the action. So, it's sort of a configuration
option for the action. Sometimes I want to insist that
a particular hidden field be present and/or insist that
its value meet some criteria. If it doesn't, I want to
produce an error message. I notice that Rails has its
own excellent error messaging system. I'm wondering
whether this is something I should try to tap into or
should I attempt to create my own? Any guiding
comments would be appreciated. Thanks in advance.

     ... doug