By putting () after the function name, you’re actually trying to run it right there in your first line.
Instead, you should just use the function name without running it:
if ($.isFunction(myfunc)) {
However – If myfunc is not a function and is not any other defined variable, this will still return an error, although a different one. Something like myfunc is not defined.
You should check that the name exists, and then check that it’s a function, like this:
if (typeof myfunc !== 'undefined' && $.isFunction(myfunc)) {
Working example here: http://jsfiddle.net/sXV6w/