User blog comment:Toothless99/Things I noticed/@comment-86.165.50.61-20110107191745

"No-one knows what a Night Fury looks like, but when they first see Toothless for real, they go 'Night Fury'. How do they know it's not a totally new breed? How do they know it's a Night Fury?"

Having watched the director's commentary, every dragon has a unique type of fire. I've also noticed that Night Fury attacks are accompanied by a kind of high-velocity scream noise. Toothless makes this noise and fires one of his fire/plasma balls at the cage bars when he turns up to rescue Hiccup. I'm assuming this is how they make the connection.

Also, Gobber's tone of voice is one of discovery - he's recognised the fire and noise, and is awed at the first sight of a Night Fury.