Ok this is an easy question, with an odd idea for the answer.

This guy will be shown pictures of 20 different creatures. One will be a dragon and one will be a dog. The rest will be creatures that may almost look like a dog or a dragon. The guy who will be looking at the pictures for what ever reason does not know what animal looks like what. Your job is to tell him what to look for so he can tell what creatures is a dragon and what creatures is a dog.

EDIT: My discriptions of the pictures is a little weak, I have been asked to add more information.

Each creature has a number of pictures taken of it. At different angles and of some close up notable body parts. The pictures are taken to where the size of the creature can not be worked out due to scale. And each set of pictures are of a different animal. So there may be a cat, snake, rat, crocodile and so on. This is off something that has been bothering me for years. When people say "dragon like" creatures are found around the world, what are they using to put these creatures together?
/EDIT

It can be in whatever form you want to use. I am still working on it myself, but I plan to use a point by point list.

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The victories from those skilled in warfare are not considered of great wisdom or courage, because their victories have no miscalculations
- SUN-TZU

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

Yea, that is kind of the reason I thought of it. A common used line is "Dragons are found all around the world in the myths and folklore, so they must exists." So what does a dragon look like then? If people from all of history and from all around the world see somewhat the same creature it ought to look about the same. Kind of like a dog looks like a dog. Even when you bring different breeds into the factor all dogs have a common look.

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The victories from those skilled in warfare are not considered of great wisdom or courage, because their victories have no miscalculations
- SUN-TZU

true all dog breeds look different slightly as well but are close enough alike that when you look at a dog you know that you aren't looking at a cat. Multiple breeds could explain why everyone has a different look to dragons in general. The question is most assuredly "What do they all have in common?". A few things come to my mind when posed this question. 1 a lot of the tales about dragons mention wings or the ability to fly which implies wings. 2 most tales agree on scales as generally the type of skin. 3 most agree on size as well... large reptilian creature.

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May you never feel a Forgotten Dragon's Ire I miei Amici.

I suppose we will have to make a list of common features of dragons.
First will have to be scales, there may be a few exceptions, but...

The power of flight wont be a useful thing to look for if these pictures are done in such a way as that you cant see the scale of the craeture. If it has bat wings, then yes it is a dragon, for sure, but if it doesnt, that means nothing because Chinese and Japanese dragons generally dont have wings. And you wont be able to see if its floating or not.

Would slit puplild eyes be something to look for? Dogs definitely done have those, but you would have to assume that most dragons have them, but we dont know for sure.

Dragons can also exist in alternate planes of reality. They can still be dragons if not in a physical body. I enjoy the compilation of information about dragons linked to this board - "from snout to tail"

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5 Inner Rings