itcl::is tests whether a variable is a class. Returns a boolean true or false.

  package require Itcl
  itcl::class helloworld {
    public variable owner "No-one"

    method greet {} { puts "Hello World from $owner" }
  }
  itcl::class goodbyeworld {
    public variable owner "No-one"

    method greet {} { puts "Goodbye Cruel World from $owner" }
  }
  helloworld  h1
  goodbyeworld h2
  h1 configure -owner Me
  h2 configure -owner You
  h1 greet
  h2 greet

The following reports that both h1 and h2 are itcl objects.

  puts "Variable h1 [itcl::is object h1] h2 [itcl::is object h2]" 

This snippet reports that h1 is a helloworld, but h2 is not.

  puts "Variable h1 [itcl::is object h1 -class helloworld] h2 [itcl::is object h2 -class helloworld]"

A similar method "isa" has been added which returns a logical:
   h1 isa helloworld
    puts "Hey [h1 isa helloworld]"
 Hey 1
    puts "Hey [h1 isa goodbyeworld]"
 Hey 0
    puts "Hey [h1 isa sausage]"
 class "sausage" not found in context "::helloworld"

And see also info which returns a string:
    h1 info class
 ::helloworld