I think that generally, but not always, pastured eggs have a darker yolk color. More importantly though, the yolk should be firm and stand up, when you crack the egg into a pan for example, the yolk should be almost a half-sphere rather than a flattened blob. The yolk should hold together well and not be punctured easily, and if you separate them, the white and yolk should separate readily. Also, generally, pastured eggs are smaller with a lower ratio of white to yolk than the huge supermarket eggs.
The yolk should taste very creamy and eggy. This is subjective, since any yolk tastes creamy, but you should be able to taste the difference, I find pastured eggs are to supermarket eggs as cream is to half and half.
I agree with other posters that they can probably fake the yolk color somehow, but they can't fake these other things. Also, sadly, not all pastured eggs are really better. I bought pastured eggs from a local farm for a while, and later learned that the chickens were fed standard chicken feed with a little bit of extra soy added, and "no animal products or by-products". I am not sure if that means no bugs, or what, but I'd actually prefer that the chickens ate an omnivorous diet, since that is their natural diet, and I think it produces better eggs.