There are a few signs you can look for to determine if your British Shorthair is getting enough exercise:

1. Weight: One way to tell if your cat is getting enough exercise is to monitor their weight. If your cat is at a healthy weight, it’s likely that they are getting enough exercise. If your cat is gaining weight, it may be a sign that they are not getting enough exercise.
2. Energy level: A cat that is getting enough exercise should have a normal energy level. If your cat seems lethargic or inactive, it may be a sign that they are not getting enough exercise.
3. Playfulness: Cats that are getting enough exercise will typically be more playful and energetic. If your cat seems less interested in playing, it may be a sign that they are not getting enough exercise.
4. Behaviour: Cats that are not getting enough exercise may exhibit behaviours such as excessive grooming, scratching, or destructive behaviour. If you notice any of these behaviours in your cat, it may be a sign that they are not getting enough exercise.

Overall, the best way to determine if your cat is getting enough exercise is to pay attention to their behaviour and energy level, and to consult with your veterinarian if you have any concerns.