As someone who is not a vet, I'm right there with you. Most times I can tell what's going on or can at least determine within a couple of hours when my dog ate something that didn't quite agree with him.
What I can't do is determine how things are going inside him. For that I have an awesome vet (actually, two in the same office who I've been going to for years), who let me know whether I should be worrying about one thing or another.
Because dogs age a lot faster than us, it makes sense to follow the common recommendations of going twice per year for a check-up. This covers not only catching early symptoms of outer facing maladies like arthritis, but also getting blood work done to catch issues brought up by weakening immune systems.
Use the visit as an opportunity to discuss changing dietary and exercise changes you should consider as your dog gets older. Simple preventative measures put in place can increase the quality of life for your pooch.
Also, ask about new and/or alternative therapies like acupuncture or hydrotherapy. The Internet is a great place for getting introduced to new ideas, but nothing beats a trusted veterinarian when it comes to getting the qualified facts specifically for how they relate to your dog and you.
Photo Credit: Tobyotter