Generally speaking, cogs will have some amount of information stamped on them (like "12T"). Still generally speaking, the side with the writing on in should face to the outside (i.e. drive side) of the rear wheel. This is good for current Shimano, Campy, Mavic, and SRAM cassettes. Don't flip them around. Replace them when it's time. I replace cassettes every three to five chains. I check them with a Rolhoff cassette checker. And, any time I replace the cassette, I take a really hard look at my chainrings. I know which chainrings are getting the most usage and I try to replace them when they need it, also. Most of the time, when you replace the cassette or the chainring/s, you'll see a dramatic improvement in the quality of the shifting. It will often surprise you (how much better it gets).