Howto Reverse-Engineer a REST-Webservice
You can use network analyzer tool like WIRESHARK to capture data sent on networkRead more about it on wireshark site
For example i hit login web servicedata captured by wireshark is as:
In above image you can see data sent on HTTP protocol
1.) method POST
2.) Under MIME infofor example
Content-Disposition: form-data; name="phoneNumber"\r\n\r\n
name of parameter is phoneNumber
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Here you can see fiddle captured data:
1.) Click on GO bottun, after that it starts capturing requests 
you can use
network analyzer tab under inspect element option in google chrome
where you can see all the data and scripts loaded for that webpage, for example check this screenshot, where i am trying to see which google api is used to find coordinates for the given address.
in this you will get all the information required.