Well, the sickness signs are variable from animal to animal. There are many obstacles in identification of the disease as animals lack the subjective expression of their feeling but definitely, you can observe the signs visually.
Signs are of two main and broad divisions;
1. Direct signs: Those signs which directly reflect the disease e.g. Loose motion, having varying levels of diarrhea indicate the disease of Gastro-intestinal tract.
2. Indirect signs: There are a large number of signs like
Shivering,
Inactive
Loss of appetite,
Lethargic,
Irregular breathing,
Lazy
Nervous signs (restlessness, round circles, behavioral differences from the normal routine)
These types of signs may relate with various problems. So these can be supported with regular veterinarian checkup, or a regular laboratory diagnosis (e.g. Analysis of the blood) of the pet.