Is there a way to see http header forwarded to another url ?

Assuming the traffic is either (a) not encrypted or (b) the F5 is decrypting it then yes, using an HTTP profile and iRule or Local Traffic Policy.

See the iRule HTTP::header CloudDocs page for more details.

when HTTP_REQUEST {
  if { [HTTP::header exists "myHeader"] } {
    log local0. "The value of myHeader is [HTTP::header value myHeader]"
  }
}