How to programmatically login to webpage without JSON?

I'm trying to make android app which will login and send some data to the webpage which was developed by another person and I don't know if JSON is used in this page's php scripts. Or if it was used I don't know tags names and info which it transfers. I'd like to know if there is a way to parse login credentials directly into page's HTML form without listening php on this website, or maybe there is a way to know if this page is using JSON . I want to make this without using webview on latest android API 23.

P.S I don't have FTP access to this webpage.