Initially, you should notice the pimples drying up. They can leave red marks behind that can last for months. The red marks are not affected by the acne treatments because they are simply remnants of the acne that was present earlier, rather than active acne, and they should not be confused with true acne lesions. They should be touched or picked at as little as possible to allow the skin to recover more quickly.
With continued treatment, the next thing that you should notice is that you get fewer new lesions and that the lesions that do appear clear more quickly with less redness or scarring, if any. Over the next few months, the red marks may turn brown before they disappear, or fading creams can be prescribed to help them fade faster.