How to tell when a slight dog bite on your arm is getting infected?

The classic signs... Of infection are redness, heat, swelling, and pain. If any of these are increasing rather than improving with time, you may be developing an infection and should see your doctor.
Several ways. Increasing pain, redness or streaking of your arm, drainage, increasing swelling. Dog bites require immediate attention and appropriate antibiotics. Get evaluated asap before it becomes a more significant problem.