Everything that the client passes to the server can be self-configured. You cannot trust anything, except for an IP address. So you cannot check the header data, if it's a proxy or a normal client.
By the way: It's the intention of a proxy not to show being a proxy :)
For sure, you could take the requester's IP address and send a http request you would send to a proxy. If it reacts, it may be a proxy otherwise, it's a normal client. This method would be very expensive and not reliable. If the proxy your server requested was behind a firewall, you would get no answer and think that it is a normal client.
