It will be difficult to know if your phone has water damage as the internal LDIs (Liquid Damage Indicators) are hard to access as they are inside the phone itself.
However, iPhones usually also have two external LDI's. These are located in the headphone jack and the dock area. To see them, you will need a torch. You should be able to see either a red or white dot when you look inside. Red indicates water damage, whereas white will indicate no water damage. If both dots are white, then your iPhone is fine.
If your iPhone has two red positive dots however, then it is likely your phone has been affected by water damage. Although, there are two more LDI's on the inside which will tell you more that are tricky to access as you will need to disassemble your iPhone.
If your iPhone has one red dot and one white dot then it will also need to be diagnosed for water damage by the inside sensor too.