You are on the right track, for sure. What I typically do is dive into the PageReference and verify all the individual pieces of it including checking for null, then asserting the URL, and finally the parameters that are part of the PageReference. Below is a mock test method of what I typically do when testing my PageReference methods.
static testmethod testMyMethod(){
campaignmanageclass obj=new campaignmanageclass();
PageReference pageRef = obj.linkIsClickedAction();
//Beginning of System Asserts, verify pageRef and URL First
System.assertNotEquals(null,pageRef);
System.assertEquals('/apex/StandardCampaign',pageRef.getUrl());
//Verify the pageRef parameters as well
Map<String,String> pageParameters = pageRef.getParameters();
System.assertEquals(1,pageParameters.values().size());
System.assertEquals('recordId',pageParameters.get('id'));
}
As an FYI, the getURL() method of the PageReference doesn't always return the parameters in the order you might expect them, so instead of constructing the URL like new PageReference('/blah?parm1=val1&parm2=val2'), I construct the base URL first, and then use the getParameters() and put the variables into the PageReference that way.
This makes the unit testing aspect easier, because I can then assert that the URL is accurate and then I can use the getParameters method to walk through and compare the key / value pairs without respect to guessing the order.