how can I ensure web client's request data is credible?

there need some ‎explaining of the 'credible'.

Scene: I postulate A as my web server, B as third-part server and C as client by Browser.Please read on (I'm sorry, it's difficult present by code)
The C post a request to B(third-part server),B response C with some data and than C post the data to A.
Question: as above postulated, A is my server and how can I ensure client's data is credible.

It seems difficult in web environment because all of the client code could be seen and request also be created by client.
In third-part server support a direct feedback to my server,but there haven't.It means, I will get the data by client, does have some authentication between client and server or other ways to make the data is credible?
Thanks reading.Any advice hope!