Since getElementsByTagName returns a list of nodes, simply check for the length property this way:

Code:
Dim xNodeList   As IXMLDOMNodeList

Set xNodeList = xDoc.getElementsByTagName("MyNode")  'returns list of nodes with this name

MsgBox xNodeList.length '0 if no matches
where xDoc is your MSXML.DOMDocument variable.

Check out this URL for more info, that's where I'm learning MSXML basics.

http://www.thescarms.com/XML/DOMTutorial.asp