How can I tell if my cat has a bladderstone?

My cat Joey who is 11-12 yrs old has been acting weird lately. 3 days ago he peed on the floor when he always goes to the litterbox. Today he peed again and I caught him sitting weird with one leg up. I watched him for 15 mins scared out of my mind because he looked to be in pain and was breathing heavy. He then proceeded to lick himself, stop, and leave his leg up in those 15 mins. He normally goes to the restroom fine, never sits weird, and never growled or hissed at me; even after messing with him. He does sit around a lot and is very lazy; he's not obese and is a very friendly. I looked up to see if there might be something wrong with him and my sister said that he might have a bladder stone. Is she correct or is something worse with my cat then that. And if he does have a bladder stone how do I help him?

He needs to be seen by a vet.  He could have a UTI, crystals or bladder stones, he needs to be diagnosed by a vet.  If he has any of these he will need a round of antibiotics, crystals require a specific prescription food to dissolve them then a maintenance diet.  Stones can sometimes be dissolved with the same food that is used for crystals but often surgery is needed to remove them.