You can use the widget valid boolean from the form to check if is valid, if not you have the possibility to send a feedback message. Also, you can see input by input if is valid, or put valid as false and give a validation message too.

Just don't forget to put this true, this will verify if everything in the form has unfilled mandatory inputs.

Check this article: https://success.outsystems.com/Documentation/11/Developing_an_Application/Design_UI/Forms/Validate_the_fields_of_a_form?_gl=1*qeadpa*_ga*MTQwNDkyNTkzMS4xNjQ3MTA5ODIx*_ga_ZD4DTMHWR2*MTY0NzQ0MTY0OS41LjEuMTY0NzQ0MzIzMy41Mw..

Kind Regards,

Márcio

Hello Mahesh, 

Adding to marcio answers

Please check in the course
input validation for form video link : https://www.outsystems.com/training/lesson/1778/input-validations
Exercise Link : https://www.outsystems.com/training/lesson/1776/input-validation-exercise

If you referencing this Exercise  end to end then you will get your requirement as well

Hope this will helps you :)

Thanks and Regards,

Akshay Deshpande