Firstly you need to find all the spots where urine might be on your concrete patio. Sometimes this is easy if you know your dog only pees in a particular place. If you have recently moved into the house, have a new pet who pees in different locations, or are not confident you know exactly where the urine is, you will need to be careful to identify all the urine locations on the concrete.
The best solution is to obtain a blacklight, which is a type of UV light. Remove your patio furniture and scan the concrete patio carefully in sections. Old urine stains will show up as green, yellow, or blue marks. As liquid can spread through concrete, make sure you do a thorough scan of the patio. It is worth scanning the walls and sides of the patio too. Marking the spots you find with chalk can help you remember the locations.