How To Tell If A Form Object Exists?

I have a form, that depending on some conditions MAY have a certain drop down box in it.

I would like to have a javascript function that checks to see IF the drop down box exists? ie: Does the javascript object exist?

Sort of like this:

function resetMenu1(){
//Test to see if dropdown exists.
if exist(document.menu.file.value) {
document.menu.file.value = "Yes!";
} else {
alert("Box does not exist.")
}
}

Anyone know how to do this? It's the "if exist" part I don't know how to do.