If some random guy came around asking about Kara he would be rather suspicious of him.  But then when Dick asks, "She told me if I needed help to come here, who are you?"  "I'm her cousin!  Now who the hell are you?!"  That's when it clicks for Dick and he goes, 'ah shit, this is Superman!'  At which point Kal tries to piece together who Dick is, and knowing the Nightwing story makes the connection as well.  Then go from, 'who the hell are you'  to 'oh...have a beer'<br/>