Is the payment processed right away?

Is the payment processed right away? Can we tell if credit card was declined immediately?

Yes, the payment should be processed right away.

If your respondents should enter an invalid credit card, they will not be able to submit your form.

Just to add, the CardConnect integration has a Sandbox Mode that you can use to test it first.

https://www.jotform.com/help/468-How-to-integrate-CardConnect-with-your-forms