How can i use some undocumented api?

Hey guys, just curious. I know that there're some platform has various api endpoints, but just some of that are documented, others are not. I was just wonder that if you guys still want to use those undocumented API, how you guys know what kind of payload, headers, json v.v.v.v that undocumented API needs.
For some specific actions on some platform they might call those API, i want to use it, but can't find an instructions or anything about it. Some people said i shouldn't use those APIs because they're undocumented for some reason. But we can still see it in the develop tool. Any idea how to use those API ? Appreciated guys!

Generally testing them and trying to use common things that are used in a lot of apis

Test with postman

You should get in touch with the developer or business that created the API and ask for access or documentation, If that isn't an option, it could be better to look into substitute products or services that provide the functionality you want via supported and documented APIs.

p3n7357 said:

Generally testing them and trying to use common things that are used in a lot of apis

Click to expand...

This, essentially.

Additionally, if you've got access to some application that calls the API, you could monitor that in order to get an idea of how it works.

You must check requests on web through proxy and see headers, body and etc of requests, or if its mobile app you can use reverse engineering