Checking chain wear is relatively easy, if you have a 12" ruler. Lay the chain flat, and place the ruler on the chain, with the 0" mark at the edge of one of the chain pins. Look at how the 12" mark lines up with the chain pin at the other end. If it's more than about 1/16" off of the edge of the pin at that end, consider replacing the chain, which will also likely mean replacing the cluster. If you're not having any problems with shifting or the chain jumping (especially on the smaller cogs), there's no great hurry, though. Chainrings don't wear anywhere as quickly as chains and clusters, so unless you see that the teeth have an obvious "shark tooth" profile, I wouldn't worry about it. And unless the derailleur pulleys are cracked, broken, or binding, I wouldn't worry about them either.