I just had a look to SOAPUI. We do something similar with unit tests. You can find them in your Moodle folder /webservice/simpletest/testwebservice.php
There you can choose to test protocols and functions.

Note that testing Moodle web services is very complex because:
1. the web service functions can have very complex parameter structure
2. the web service can perform complex operation related to the way Moodle is set up and the content.

So I guess you will be better coding unit tests than using SOAPui that can not internally communicate with Moodle. Of course if you have not access to the code, SOAPui seems a good tool. But I don't know much about it, hopefully someone with better knowledge that me can help.