The scene in "Smith & Jones" when they are on the moon and Martha's talking about the cousin she lost at Canary Wharf. You could tell that the Doctor is sizing her up. Ten was doing a mental checklist on Martha. Ticking all the boxes. Donna should have been more specific in her advice on having someone to stop him. For example she could have said: "Be honest about how Rose really left, and make sure she doesn't fall for you!. You big dumbo!".