It's the indexOf property you're looking for.
As you can probably guess, it returns the index of a particular value in a particular array, OR it returns -1 if the value isn't in the array.
Simple example:
Code:
var dogs:Array = ["labrador","pekinese","corgi"];
trace(dogs); // labrador, pekinese, corgi
var newDog:String = "spaniel";
if(dogs.indexOf(newDog) == -1) // if newDog isn't already in the array
{
dogs.push(newDog);
trace(dogs); // labrador, pekinese, corgi, spaniel
}
else
{
trace("No, thanks. We've already got a " + newDog);
trace (dogs); // labrador, pekinese, corgi
}
Hope that makes sense.