Edit in JSFiddle

function myfunction()
{}
if (typeof myfunction !=='undefined'  &&  $.isFunction(myfunction) ) {
alert('myfunction exist');
}
else
{
alert('myfunction does not exist')
}
if (typeof mycheck !=='undefined' && $.isFunction(mycheck)) {
alert('mycheck exist');
}
else
{
alert('mycheck does not exist')
}