How do I locate .mli files from OPAM modules?

I have some code that opens a module (Async) that I have installed through OPAM. I would like to see the signature of this module, but where do I find this?

I have traversed the Async github repo but it was very unenlightening. How do I approach this problem?

This is the code I am looking at:

(* This file is in the public domain *)

(* open Core.Std *)
open Async.Std
(* open Cohttp_async *)
 
(* given filename: hello_world.ml compile with: 
   $ corebuild hello_world.native -pkg cohttp.async
*)
 
let handler ~body:_ _sock req =
  let uri = Cohttp.Request.uri req in
  match Uri.path uri with 
  | "/test" ->
       Uri.get_query_param uri "hello"
    |> Core.Std.Option.map ~f:(fun v -> "hello: " ^ v)
    |> Core.Std.Option.value ~default:"No param hello supplied"
    |> Cohttp_async.Server.respond_with_string
  | _ ->
    Cohttp_async.Server.respond_with_string ~code:`Not_found "Route not found"
 
let start_server port () =
  eprintf "Listening for HTTP on port %d\n" port;
  eprintf "Try 'curl http://localhost:%d/test?hello=xyz'\n%!" port;
  Cohttp_async.Server.create ~on_handler_error:`Raise
    (Tcp.on_port port) handler
  >>= fun _ -> Deferred.never ()

let () =
  Command.async_basic
    ~summary:"Start a hello world Async server"
    Command.Spec.(empty +>
      flag "-p" (optional_with_default 8080 int)
        ~doc:"int Source port to listen on"
    ) start_server
    
  |> Command.run