Most people would think, if you can see a little Tread on your tires, you are ok. This is incorrect. There are many ways to check your tread dept. The penny test is one, but I recommend what is called the treadwear indicator bar. These are bars that can be found in the bottom of tread grooves in different spots around the tire. When these bars are flush with the adjacent ribs the tire needs to be replaced per the manufacturer.
Also, it is a good idea to periodically check the inside wall of the tire. We find that sometimes the tires look good on the outside but have the radials exposed on the inside. Maintaining proper tire pressure will allow the tire to function properly.
A quick word about the spare doughnut tires. P the manufacture if the spare tire has ever been put on the car it should be disposed of. These small spare tires are designed to get your out of a bind. These are very low speed tire. Not designed for normal highway speeds.