[jQuery] How do I return false if no elements found using XPath?

[[EMAIL PROTECTED]]

I am trying to find a particular parentElement without knowing the ID.
If not I need to find a different element. SO:

var parentXpath = "[EMAIL PROTECTED]@id="+obj.id+"]/..
[EMAIL PROTECTED]";

if (*noobjectfound*) {
find new object
}

How can I do this?