When sketching portraits of any kind (people or pets), viewing the image through a mirror from time to time will expose errors.
Sketching upside down is always a good way to get over an area of difficulty. Turn the image and the support upside down. In fact, many artists do this as a warm up before beginning a project. This exercise stimulates the use of the right side (the creative side) of the brain.
After sketching, but before you begin to paint or shade, always remember to identify your light source and look for your areas of tone (highlights, medium areas, shadow edges, cast shadows and reflected light areas). Make sure you understand how the light is affecting your subject/s.