Thats really interestinng. If I was to tell the guy to look for specific features so that the creature can be identified, the first thing I thought of was Hair. Tell the guy to look for fur, then it is more likely to be a dog. But then theres a problem, because some people draw dragons with fur, or feathery fur, so......
Then I thought about the feet. A dog has a paw, with four main digits, and one smaller fifth one further up the leg, all have claws, small and slightly curved. But again, ive seen people representations of dragons that have paws like lions, which are similar to dogs, except they have retractable claws. So, all I can think about is this poor guy getting very confused.
And you said that size comparison is out of the question, so that makes it more difficult.
I suppose if you ask the guy to just look for scales, that would be the best bet for a dragon creature, because dogs dont have scale at all!
Hmm, I will have to think more on this