How to debug post forms from magento payment module?

Question

PHP

I'm making my own shop on magento. I write extensions for a payment gateway for my needs. I have a method that post'om sends a form from redirect.phtml to the gateway address with listing the contents of the inputs, and I (the buyer who regrets parting with $$$) is redirected from the checkout to the same place. So - something is wrong or wrong is transmitted to the payment gateway and it is sad from this.
What would or how would I see which line (what is in it) is transmitted to the gateway? What to debug?

Answer 1

in the browser in DevTools Network tab