New Opacity Question

I want to amke something fade out, if and only if it is already set at 100% opacity.

I need to make a function that will allow me to do this.

Here is my current code:

//Time=millisecond Start and End = Amount of Opac to start with. Id = which div to affect
function opacity(id, Start, End, time) {
//frame speed here
var speed = Math.round(time / 100);
var timer = 0;

//
if(Start > End) {
for(i = Start; i >= End; i--) {
setTimeout("Opac(" + i + ",'" + id + "')",(timer * speed));
timer++;
}
} else if(Start < End) {
for(i = Start; i <= End; i++)
{
setTimeout("Opac(" + i + ",'" + id + "')",(timer * speed));
timer++;
}
}
}

function Opac(opacity, id) {
var obj = document.getElementById(id).style;
obj.opacity = (opacity / 100);
obj.MozOpacity = (opacity / 100);
obj.KhtmlOpacity = (opacity / 100);
obj.filter = "alpha(opacity=" + opacity + ")";
}

How would I write a function that would allow me to fade out if opacity was already 100, otherwise it does nothing?