Are your parents a part of a church? If so, I'd contact the church and ask them to have someone drop in and check on them.
You might also check if there are elder care services in your city/county/state. This would have a fee attached, but it might be worth it for your peace of mind.
I'd also be sure I had my cell phone at all times and that this number was on my parents' refrigerator, or a speed dial on their phone. And then you want to be sure that you have phone numbers in your cell phone for contacting those near your parents' home for whatever the trouble may be (hospitals, doctors, pharmacy, even a jack of all trades repair guy).
Another thought....summer time you'll have a lot of college students home....see if you can find a nursing student, or med student even...who might not mind stopping by to say hello each day for a few dollars. Of course, this is more risky if you don't know the person, but there are ways to do a little background checking on someone, though I'd still not place an ad or something, I'd try to get a neighbor's niece or the mailman's nephew....or even the girlfriend of your co-worker's brother, lol....some personal connection would make me feel better.
Last resort....if you, for instance, try calling home to check on them and they don't answer and you're worried.....you can always call the police and they'll do a well check. It's not considered a high priority unless you have something to make you fear they are in immediate danger....but they'll drive by and ring the bell and check to be sure they're ok and haven't fallen or something. It may take them several hours before their schedule allows this, so it's not the best solution, and I'm sure they won't do it daily. It's really a "I'm very worried and don't know what else to do" solution.