The front sprocket wear occurs on the forward side of the teeth, as that is the side which exerts pressure against the chain. As a result, they appear to be bent, or hooked, forward. The opposite is true for the rear sprocket. There is a spec for chain wear in the service manual, usually given as a number of millimeters between a certain number of links. An easy way to check is to pull the chain back away from the rear sprocket. If you can see daylight between the chain and sprocket, (generally about 4 or 5 mm), it's worn out and the recommendation from industry professionals is to replace chain and both sprockets as a set. Chain and sprocket wear is one of those things that sneaks up slowly. You don't realize how bad it is until you put on a new set, then all the smoothness and power of your new bike is magically back.
Contrary to popular belief, chains do not strech. They wear at the pins, where you can't see it. Just a thousandth of an inch multiplied by over 100 links will make a chain seem to be "streched" from its original length.
How long a chain lasts varies greatly by how you ride. Some who are easy on the bike may get 30000 miles, but if you have a heavy throttle hand you can trash one in 12000 miles, maybe less.
Stay away from aluminum sprockets, they wear out very quickly and it's not worth the savings in weight, unless you are racing.
Of course, keep everything clean and lubed every few hundred miles, more frequently if you ride in rain.
I don't want a pickle, I just want to ride my motorsickle. A. Guthrie.