How to check if an element exists in the XML using XPath?

Use the boolean() XPath function

The boolean function converts its
argument to a boolean as follows:

• a number is true if and only if
  it is neither positive or negative
  zero nor NaN

• a node-set is true if and only if
  it is non-empty

• a string is true if and only if
  its length is non-zero

• an object of a type other than
  the four basic types is converted to a
  boolean in a way that is dependent on
  that type

If there is an AttachedXml in the CreditReport of primary Consumer, then it will return true().

boolean(/mc:Consumers
          /mc:Consumer[@subjectIdentifier="Primary"]
            //mc:CreditReport/mc:AttachedXml)