Typically hearing aids have some sort of marking indicating which side they are meant for. If there is any writing on the device or color on the earpiece, red is for the right, blue is for the left. If you can’t find any markings on the hearing aids, it is time to do some detective work. If the devices go behind the ear, hold the hearing aid with the part that sits on top of the ear facing back and the earpiece facing forward. If the earpiece is pointing to the left, that is the right ear hearing aid, and vice versa.
How about if the hearing aid is an In-the-Ear style? If there is any writing on the device it will face forward, and then the opening on the device where the sound comes out will face into the ear. Also, if there is a vent (a hole that runs all the way through the hearing aid), the vent should be on the bottom and the hole where the sound comes out should be on top. Once you have figured out which device goes in what ear, it is a good idea to use nail polish to paint a red mark on the right ear so that you can distinguish them in the future.