Although squirrels are rarely found to have rabies, it has been known to happen. If you are bitten by a rabid squirrel, your odds of contracting the disease are slim, unless the animal was already appearing very ill. Nonetheless, once you suspect that a squirrel has rabies, you need to be able to tell the signs right away, especially if the animal is a family pet.
If possible, determine if there have been any changes in the squirrel's behaviour. If the animal begins doing things outside its normal routine (not related to cyclical behaviour such as pregnancy or mating), this may indicate the presence of rabies. Examples of behavioural changes include increased sensitivity to light, sound and touch. A once calm and curious squirrel may become vicious, combative or aggressive. Alternately, they may be shy, withdrawn and timid -- again, this is only cause for concern if it's outside their normal mode of behaviour.
Observe the animal for the later stages of rabies, where it will randomly attack anything it can, including both humans and animals, and sometimes even inanimate objects. During this stage, the squirrel's behaviour will be wildly out of character, and it may eat non-nutritive substances such as pebbles or inedible garbage. It may also have extreme changes in sleeping patterns, such as sleeping all the time, or rarely closing its eyes at all.
In the final stages of this disease, paralysis may take hold, with the animal barely able to move its limbs at all. This is where you might see the squirrel exhibiting classic rabies signs, such as foaming at the mouth. Coma and death inevitably follow shortly thereafter.
Most undomesticated squirrels are naturally wary of humans, and will not approach them unless forced to in some way. If a squirrel is running directly at you, or approaching you without provocation (such as the offering of food, which you should not do), there is a good chance that it is infected with rabies.
If you suspect that a squirrel has rabies, contact your local health department or veterinarian before attempting to treat or destroy the animal on your own. Do not feed, pet or approach the animal until it has been deemed safe to do so by professionals. Prepare yourself for the fact that if the animal is a pet and has rabies, it will have to be put down for the safety of yourself, other family members, pets and the general public.