Ti

//Page reference to your VF Page
PageReference pageRef = Page.TestPage;
Test.setCurrentPage(pageRef);
 
//Pass necessary parameter
pageRef.getParameters().put('Id',id); 
 
//init controller 
CustomCtrl objCtrl = new CustomCtrl();
 
//Call pageRef mymethod
PageReference objPageRef = objCtrl.mymethod();
 
//Put system asserts
System.assertEquals (null,pageRef);