On many cars you can inspect the brakes without even removing the wheel. If your car has alloy wheels with spaces in the middle, you can get a proper assessment done by just peeping through the hole at the goodies. Whether you can peep through your wheel or you have to take the wheel off, be sure you have a clear view of the brake pads and the big shiny disc.
Let's inspect the disc first. It should be shiny from the inside to the outer edge, and fairly uniform. Don't worry if you can see slight lines in it, this is normal wear. However, if there are any rough spots or pronounced grooves in the disc, you should replace your brake discs. Brake discs should always be replaced in pairs so that your car's driveability and safety are not compromised.
Now take a look at the pads. You'll have to peek up to see them, but if you follow the surface of the disc to the top, you'll see the outside pad touching the disc. If there is 1/8" or less remaining on the pad, it's time for new ones. That's about the height of two pennies stacked. Don't worry, brake pads are cheap and replacing your brake pads is a no-sweat job.
Finally, take a look at your brake lines. Rubber coated lines should be soft and supple, not cracked and rigid. If you see cracks in flexible brake lines they will need to be replaced. Also be sure to inspect the hard, metal lines. These can become corroded, especially in snowy regions where chemicals are used on the roads.