Rinse your dog thoroughly. As long as you see dirt or soap bubbles in the water coming off of an area, keep rinsing. You can use the same method you used to soak the dog's coat before shampooing. If your dog is too afraid of running water or the bath in general and can't be done on your own, there are veterinarians who can give proper sedation, not too much, to allow you to groom in a couple of hours or can groom the dog themselves. Touch the dog all over to feel for any shampoo especially the chest area and in between the legs they are hard to get. To test it, rub the fur in between you fingers and pull softly, if it feels squeaky then it's clean.
Please note that regardless of pet type, we always put safety first. Sometimes pets can stress in new situations. If we feel your pet is too stressed or showing any potential health concern, we may opt to conclude the grooming appointment in order to do what's best for them. Your stylist can help recommend a grooming service for a smooth introduction, such as a Mini Make-Rover. For any potential health concern your stylist will refer you to your veterinarian.