Can you qualify exactly what you mean by 'validate?'

The easy thing to do would probably be to just do a 'dummy' E/M API query, like a deviceUserQuery with an empty deviceName. You will get an error, but if it's not an authentication error, you know the credentials work.

To positively verify that the user has permissions, you can use the Administrative AXL service's getUser request. Among the fields returned is a associatedGroups, which will contain a list of all the User Groups. You would be looking to see the list contained the 'Standard EM Authentication Proxy Rights' group.