Hi Boopathi,
You should use 'Count' function to determine existence of a Node in a XML.
The count function returns the count of the occurrence of a field or node in the XML message. Hence in case a field exist in a message, the count return a value greater than Zero and if it doesnt exist, the value returned should ideally be zero.
Reference: To Exist Or Not to Exist is the Question - Process Integration - SCN Wiki
Regards,
Nabendu.