There is a dye that you can put into the system and if not in a hidden spot , will be able to find that way . If dye doesn't work , you will need to take the car to a shop so that they can use their "sniffer" .
There is to many places fro the refrigerant to leak to tell without checking.
They sell refrigerant with a red dye, and U.V. Dye. The red dye is visible. The U.V. due needs a U.V. light shined on it to find the leak.
I would find the leak so you know where it is. Then I would take it back where they fixed it alst to get them to fix it right. But know ing where the leak is I wouod know it they really fixed it right the second time.