How to find the user exit for the T-code 'RECN'

Hi friends,

I have a requirement as-

In 'General Data with Fast Entry' tab of tcode 'RECN' there are 2 fields as contract conclusion date and contract end date.

Now, the requirement is as if entered contract end date is less than contract conclusion date then we have to display a warning message as "contract end date cannot be less than contract conclusion date".

So, how to find the right user exit to enter the code.

Plz help me.I will surely reward points.

Thanks,

Rishi