Press down with your thumb on the longest free section of each belt. If it deflects more than an inch, it's too loose. Some belts are easy to adjust. Loosen the bolt that fits through the slotted mount for the alternator or other accessory, pry the alternator against the belt, then tighten the bolt. On other cars, belt adjustment is more difficult and should probably be left to a professional.
Many late models use flat serpentine belts that are self-adjusting and last a long time.
Check it monthly at least. Look up the inflation in your owner's manual. It'll be in pounds per square inch. Remove the valve cap, press the tip of the gauge against the valve, then read inflation pressure. Do this with the tires cold, not after a long run on the highway.