There is a light and a spray that you can buy that will show up cat and dog urine. You might go to the local pet store and see if they have them in stock. I know I have saw them advertised on tv. Other than that all I know to do would be shampoo the carpet good and freebreeze it.

 They have UV lights that will show the urine, if you can't already smell it, that will show urine, blood and all kinds of stuff that you might just wish you never looked for in the first place. You can buy these lights on ebay for fairly cheap!

 you don't have to buy those special lights to find it a reg black light will tell it, i have one and that is how i could tell where my dogs had urinated when we had them in our bathroom,