What is the best way to check if the value of a function's arguments match specified possibilities?

Question

Say I have a very simple function that checks the value of an and behaves differently depending if it matches one of a number of possibilities. Here's a minimal example:

is_duck <- function(x) {
  if (x == 'duck') return("QUACK")
  else return("That's not a duck!")
}

This works well enough as long as I behave with what I pass my function. But R is dynamically typed, so it's a lawless land out there. I want x == DUCK to be a length-1 logical vector which I can throw into my branching logic, but I can't guarantee it will be. Is there a better, more type-safe way to accomplish the behavior I want?

I thought about if(identical(x, 'duck')) but that would fail in the case that x has some additional attribute, which is a little too specific (i.e., x <- set_names('duck') would return "That's not a duck!" even though it is). Is isTRUE(x == 'duck') what I'm looking for?