Look for a supplement that is good quality in terms of ingredients, has a proper website with information and sealed packaging, and isn't making outrageous claims about curing cancer or any diseases (such claims are not ethical). The most important thing when it comes to a supplement for your dog, aside from it having safe ingredients, is to have accurate dosing and palatability (or taste).
Too much of a good thing can be a bad thing, so dosing becomes very important. Especially when it comes to senior dogs, they can be susceptible to liver and kidney problems from over-supplementation as they are already losing the optimal functioning of these symptoms. Therefore, finding a 1500 mg glucosamine tablet or 500 mg calcium tablet may not be the way to go. Talk to your vet about making sure that you are getting regular bloodwork done to look for an nutrient deficiencies or excesses in your dog. Then you can find a supplement that has the right quantities to support your dog's specific needs.
The supplement you choose for your senior dog should be simple and easy to give so that you will stick with it for the rest of your dog's life. It is easy to stop giving a supplement if your dog fights you every single day, making both you and your dog miserable. You want an easy-to-swallow tablet or other form of supplement that you can simply hand over to your pet to be devoured as a yummy "treat." This will encourage both you and your pet to stick with the supplement regimen. It is very important to keep your pet on a good supplement once started, or you may lose the ground you have gained.
Boneo Canine® is liver and sausage flavored, and comes as a chewable tablet, so it helps to tackle this issue. Moreover, it is easy to crush up and mix into food, so even the most finicky of dogs will be able to take this product every day.