You should generally be able to feel the ribs with light pressure and see a waist from above, but coat and body shape can make visual assessment difficult. Sudden weight change deserves veterinary attention. Use calorie estimates only as a starting point and measure food accurately, including treats.
Use this as a starting point
Every dog is an individual. Age, medical history, breed, environment and previous learning can change what is appropriate. When in doubt about health or safety, consult your veterinarian or a qualified professional.