Is there R code to test if variable is a function

I am trying to write a function myfun in which one of the variables fun, say, is itself a function. A simple, but ludicrous, example is:

myfun <- function(fun){
              return(fun(1))
             }

In line with good practice, I want to insert into myfun a test of fun to ensure that fun is indeed a function.

But I can't find code for any such test. What should I do?