One way to tell if your dog is overweight is to look at them from above.
If you can't see their waistline, they are probably carrying extra weight. Another way to tell is to feel their ribs. You should be able to feel each rib individually without too much effort. If you can't, your dog may be overweight. The final way to tell is through a body condition score chart that takes into account body weight and shape. However, the best way to know for sure is to consult your veterinarian.