Check that it's breathing is neither too heavy or too shallow,that it's eating like usual,and if it has no issues like diarrea or blood when defecating.Get a veterinarian checkup if it seems to be acting unusual for a extended amount of time or is very sick.
If your dog (is) Sleeping alot more than usual Not eating that much Deaf Blind Not really happy doesnt jump up on the couch anymore runs into walls yelps when you touch him has any lumps
He or she may be dying or for most of these symptoms just an old dog
But old dogs can sometimes be in pain...You should take he/she to the vet, if the vet says he/she is just old and has great health, then thats a plus. If your vet says that the dog is suffering and makes the suggestion to put he/she down, remember its best for the dog. This is a very hard thing to do but its better than watching the dog slowly go through all that pain day after day.