All our door to door sales agents wear an identification badge with the npower company name and logo and their own name prominently displayed. A number of agents also wear an npower uniform (red shirt and suit and red jacket). We regularly check sales agents' appearance while they are working to ensure that they comply with these standards.
It's really important that you check to see if someone is actually who they say they are when they're right on your doorstep. Any legitimate caller will not mind if you ask them for their ID - also if you close the door on them while you verify this.