It is easy to tell the difference between a rock and a fossil—all you have to do is lick it!
Paleontologists are approached by dozens of people each year who think that they have stumbled upon the fossilized remains of a dinosaur or some other age-old creature. With great excitement, they ask which dinosaur it originates from and whether the museum would be interested in buying it from them. Needles to say, most of the time it is just a rock.
But it does bring up the question: how would one tell the difference between a rock and a fossil? First, one would start with common sense. You will never find a dinosaur egg, or any remains of a dinosaur for that matter, while you are mowing your lawn on a Saturday afternoon. Real fossils are found in particular rock formations.
But let us assume that, regardless of how you got your hands on it, you have what you believe to be a real piece of fossil bone. A rock is solid and looks the same on the inside and on the outside, but if a fossil is broken you will be able to see the canals of the bone.
Finally, you could do the "tongue test." Due to the porous nature of a fossil, your tongue will slightly stick to it if you lick it. If you feel compelled to try this, have a glass of water handy!