If your pet (especially young ones) are experiencing persistent 
vomiting or diarrhea. If vomiting or diarrhea last longer than 24 
hours after withholding food and water. This can be a sign of serious 
problems which can also lead to dehydration very quickly.

If your pets activity level decreases for no apparent reason.

Any time you notice distension of the abdomen, along with 
attempts to vomit unsuccessfully, these can be signs of bloat which 
is a life threatening condition, especially in larger breed dogs. 

Watch for signs of heatstroke! If your pet has been outside 
during extreme heat and is having a hard time cooling themselves 
off. - i.e. excessive panting, restlessness, inability to get up and move, 
incontinence, vomiting. Watch for these symptoms, especially in 
the "dog days of August".

Any time your pet gets hurt, especially involving blood loss as in 
a cut pad or injured limb, etc.

At the sign of seizures.

Total lameness - complete inability to walk.

At the first sign of possible poisoning.

When your pet's skin becomes red and inflamed and over the counter 
remedies are not helping.

If they develop is any type of growth it should be checked and 
monitored regularly change in size or irregularities.

If your pet has no desire to eat or drink for a number of days - 
or becomes lethargic for more than 2 or 3 days.