If you are ensure or would like to find out if your phone has water damage before you place your order and send your phone to a recycler then you will need to check the LDI (Liquid Damage Indicator).
Most phones will have more than one and they can usually be located under or around the battery. Depending on the brand and the model there may be differences with location, size and appearance. Most LDI's will be white if they are undamaged by water, or, red if they are damaged by water. If any of the LDI's are red then this will mean your phone has water damage. Some may also have crosses. Crosses do not always mean water damaged as the may just be stickers covering the LDI, but in some cases they may indicate water damage.