You can hide the types held by the Integer and Boolean constructors and not 
hide the variant type itself:

# module Thing : sig
  type integer
  type boolean
  type t = Integer of integer | Boolean of boolean
  val mk_thing : int -> t
  val dest_thing : t -> int
end = struct
  type integer = int
  type boolean = bool
  type t = Integer of integer | Boolean of boolean
  let mk_thing i = Integer i
  let dest_thing t = match t with
    Integer i -> i
  | Boolean b -> if b then 1 else 0
end;;
module Thing :
  sig
    type integer
    type boolean
    type t = Integer of integer | Boolean of boolean
    val mk_thing : int -> t
    val dest_thing : t -> int
  end

Then you could "open" the namespace of the Thing module, saving enormously on 
typing:

# open Thing;;

Then you can use pattern matching to determine if a value of type "Thing.t" 
uses the "Integer" or the "Boolean" constructor:

# fun (Boolean b) -> b;;
Warning: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
Integer _
- : Thing.t -> Thing.boolean = <fun>

A better example might be:

# let is_bool = function Integer _ -> false | Boolean _ -> true;;
val is_bool : Thing.t -> bool = <fun>

Of course, if your pattern catches a value of type "Thing.integer" or 
"Thing.boolean" then you can't do anything with it except hand it to a 
function in the "Thing" module.

But you can't actually construct a "Thing.t" because you don't have access to 
the hidden "integer" and "boolean" types in "Thing":

# Integer(3);;
This expression has type int but is here used with type Thing.integer

As an aside which you may well already know, convention is to use a type "t" 
for the main type of a module (e.g. List.t, Array.t, String.t) and a function 
"make" to construct it. Possibly also a function "compare" so you can build 
sets and maps over the type, e.g.:

# module StringSet = Set.Make(String);;
...

Cheers,
Jon.