Some people think that if you have one bad wheel cylinder, you should replace them both. While this isn't a bad idea, it's not necessary. You can just replace one wheel cylinder if it's leaking. Another common misconception is that you need to remove the brake shoes to replace a wheel cylinder. This is not true; in fact, I don't recommend you do. You can often crank out the self adjuster to spread the shoes away from the wheel cylinder to gain access to it.
Another note on wheel cylinders: If you find that the brake line going into it is rusted, stop trying to turn it; you could damage the brake line. Instead, dismount the wheel cylinder from the backing plate and spin the wheel cylinder off of the brake line. This way, you avoid damage to the brake line, thus saving yourself a bunch of time and money.
Diagnosing a bad wheel cylinder is fairly simple. After you remove the brake drum, pull back the dust boots on the wheel cylinder and look for fluid. If you see fluid coming out, replace the wheel cylinder. You can rebuild wheel cylinders, but as with rebuilding brake calipers, you might end up with more trouble than it's worth. Only rebuild wheel cylinders if you can't find the parts you need or the parts are too expensive. In my experience, wheel cylinders aren't all that expensive. Here's a video on replacing a wheel cylinder you might find helpful.