Keep a record of the age of the tires. Over time, the rubber wears and begins to degrade. A good rule of thumb, is to have your tires checked if they are 5 years or older. You can look for cracks and tears also. This can cause blow outs, something no one wants, and can be particularly dangerous, especially at high speeds.
Look at the tread. Try the penny test if you are unsure. Put a penny next to the tread, head down. If you can see Lincoln's head, it's definitely time for new tires. If most of his head is covered up, you are probably okay.
Evaluate any uneven wear. If you can see places that aren't wearing evenly, you may have a problem. This might include sloping on one side of the treads, or chunks or pieces missing on tread. This could be due to a misalignment, (in that case, Jake's can fix that with an alignment). Rotating tires can also help prevent uneven wear.
Check tire pressure. Gas mileage and overall comfort increase as you keep your tires inflated to the manufacturers' specifications.
There is a lot of weight resting on your four tires. Take care to ensure their durability and safety, and you will have a much more enjoyable and safe trip every time.