How to validate WebService results

How can I validate the results of a webservice which is returning an xml values which has n number of nodes.

For eg in my case when I search for the weather in US it returns all the major cities(50 for now) weather for the next one week in an xml output. I am planning to validate the data.

When I specify save returned value in parameter I am getting only the first value but there are 50 more. Is there are any inbuilt functions to get the number of rows and number of nodes etc.,

Please suggest me a direction to achieve the validation.

Thanks in advance