Bruises happen when you bleed under your skin. Since a dog with low platelets has blood that doesn't clot correctly, bruises can occur due to your pet's normal activities, like playing or going for a walk. Keep in mind that a single bruise doesn't mean your dog is sick. You may notice the dog lying around a lot, and it might not be as interested in play. Additionally, it may resist going for a walk or playing with its favorite toys. If your dog seems generally tired and uninterested, it likely needs a vet check. There are many different causes for lethargy in dogs, so don't assume your dog has a low platelet count. Its nose may feel extra wet and sticky, or you may see the mucus dripping down its nose. Also, your dog may wipe its mucus on its fur or around your home, so be on the lookout for mucus streaks. Like with the other symptoms of low platelet counts, mucus streaks have other causes, as well. Excessive mucus may lead to coughing, which won't go away easily. Your dog's cough may sound like a hacking cough or might produce phlegm. If your dog is coughing, you need to take it to the vet to find out why, regardless of the cause. This happens from blood in the gastrointestinal (GI) tract. If your dog's blood isn't clotting right, then it may have bleeding in its digestive system, which will result in very black feces. You might be very scared when you see your dog's poop change colors like this, but there's no need to panic. Your vet can find out what's causing your dog's symptoms and then treat them. Don't try to take your dog's temperature or pulse yourself. It's very easy to make a mistake when you aren't trained. Instead, ask your vet to evaluate your dog to see if it has these symptoms. It's not safe to take your dog's temperature at home, as the dog might move and get injured. Furthermore, feeling the dog's nose or ears is not a reliable way to take its temperature. Both of these serious symptoms require immediate vet care. However, your vet may still be able to help your dog recover if you get it treated in time. If it's after your vet's normal office hours, search online to find out if there's a 24-hour emergency vet service in your area.

Summary:
Watch for sudden, unexplained bruising. Notice if your dog seems lethargic. Check your dog's nose for excessive mucus. Watch your dog for excessive coughing. Notice if your dog has dark, tar-like stools. Ask your vet to check your dog for a fever or heart murmur. See your vet immediately if your dog collapses or has urinary bleeding.