Appetite loss, lethargy, vomiting, limping, or sudden behavior changes deserve prompt attention.

Healthy pets have a stable appetite, energy, and bathroom routine. Note what is normal for your animal so changes stand out.
Call a vet if you see repeated vomiting or diarrhea, labored breathing, collapse, seizures, or inability to urinate. These are emergencies.
Mild issues still matter: scratching nonstop, bad breath, cloudy eyes, or hiding more than usual. Catching problems early usually means simpler care.