I am interested in adopting a particular chihuahua that is about a year old. How do I know if it is not healthy? What are the signs?

A vet check, that's how you know. :)

I am considering adopting a chihuahua from someone and am not sure if he is healthy. Of course if I adopt him I would bring him to my vet. But I was wondering if there were any signs that I should look for

If the person has nothing to hide he/she will allow you to take the dog to a vet before adopting it.

There are many illnesses or defects that cannot be seen with the naked eye, it's always best to have a vet check.

Of course, if the current owner is a responsible owner then the dog should be up to date on it's shots and a have a health check already done. In that case, just ask to see the paper work or get the number of their vet so you can ask questions.

As the others have said you can never be certain of a dogs health without a vet check. A dog does not necessarily need to exhibit drastic signs of illness in order to be dangerously ill. There are limitless problems that you may not be able to see with just the eye, including heartworms.

If you are considering adopting from a Humane Society then the dog should have already been vet checked, however if the dog comes from a regular shelter or person then you are going to need to have this seen to yourself.