a node child

Hi,
Suppose we are located on a node. How could we test the existence of a given
child node ?

example: how to test that "eric" has a telephone and "max"  has not.?
-------------------------------------------------------
<persons>
    <person name="eric">
        <tel>003366778899</tel>
    </person>
    <person name="max"/>
<persons>
--------------------------------------------------------
Thanks.


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