How do you know when dog has fracture or pulled muscle or tore ligament in legs?

Hello I have a 9yr. old dog who just fell 3 days ago. He fell on some stairs. He was going downstairs and slipped and hurt himself really bad. He cried so much. I rubbed him, but he is still in pain. I've been carrying him up and down the stairs. What we notice is that he walks very slowly and he doesn't want to put pressure on his back right leg and sometimes the front right leg. It seems like he's hovering the back part of his back by his legs. And he seems to walk with his legs stiffly together not relaxed. When I pick him up, I pick him up from his belly that way he doesn't put pressure on his back legs. The vet gave me Rimdayl and Tramadol hcl. My dog is on Phenobarbitol and potassium bromide (for epilepsy-he has not had any seizures in 8 months thank God) and Denamarin for his liver. His liver enzymes are somewhat higher than normal. I would say today he is in more pain than yesterday. When he's laying down, I can see that he is trembling. His legs are trembling. What does this mean? We haven't done xray yet because doctor said if he can walk on his legs there must be no fractured bone. We have touched his legs and he doesn't cry. Of course I have not put any pressure on them. When he fell, it's like he slid on his belly and tried to stop with his legs. The steps are cement so you can just imagine. I honestly don't know what to do. I know he's in pain. How do you know when the dog has a fracture or pulled muscle or torn ligament? What are the symptoms? How long does it take for the pain medication to work? My doc weighs 24lbs. and with his liver problem the vet gave me a minimum amount of meds. Rimadyl 50mg once a day and Tramadol 25mg. twice a day. Does anyone know if this is ok? Please give me advice. I need to know what to do. Thank you.