Dog illnesses are just as vast as a person's, and so there are countless symptoms; Is there a specific illness that you believe is occurring with your dog? Have you already noted a change in odor, behavior, or appetite? Below is a link from the Rover blog with instructions on how to check your dog's vital signs. Perhaps you could start there? One of the most common illnesses with dogs that I've encountered is upset stomach, including nausea, vomiting, diarrhea, and lethargy. If you've noticed an odor, where is emanating from - their ears or mouth, possibly? Ear infections are also rather common and usually a dog will shake their head, scratch at and rub their ears on carpets/furniture and their ears will smell. If they have a new odor coming from their mouth, check out their teeth, especially along the gum line to see if they have a lot of plaque buildup and are due for a cleaning. Gum disease is also pretty prominent among dogs and can give them bad breath. If they have a smell coming from their rear and you've noticed that they're scooting their bottom, they probably need their anal glands released.
If you have more details, I'm sure the Rover community can help more. However, if you are concerned about your dog, you should contact your vet to make an appointment.
Good luck!
How to check your dog's vital signs:
https://www.rover.com/blog/dogs-vital...