How to validate fields on button click

do you mean to invoke standard validation rule assigned to a control or to invoke some custom (javascript) validation?

I know client is alway right (), but you should convince him validation rules are the strandard way how nintex forms are designed to work.

validation on button click is unreliable and depends on user action. if he/she doesn't click the button for any reasons, he/she might be able to submit incorrect data.

button vs. Yes/No: you could create a choice control with single option 'Validate' and let it render as button.that might have satisfied the client.

@ramanjjilu ......You know that validation rules with a submit button on the form will behave the same way. 

Let's say your....

System Manager cannot be blank --> create that as a validation rule - Msg will be "Please provide System Manager's name, it cannot be empty"

Review Comment cannot be blank ---> Same as above

Review Date --> Validate that Review date is of today's or future...if not then msg will be "Review date cannot be past date." 

Instead of Validate button, simply change the label & function to "Submit".

Now if any of the validation rule is invalid it will highlight that field and on top of the form it will display your custom message.