Not easy at first glance, however, if you look at the pattern of their markings it's not so bad. First take a
look at the common oto. Pay special attention to it's tail and the stripes on it. Now take a
look at the Chinese algae eater. It's tail has spots on it rather then the broad stripes. Also look at it's body, yes it has the stripe down it's side like the oto, however, take a closer look and you will notice that's it's not just a stripe. It's a stripe combined with spots and those spots carry over the top of it's body. The oto just has that stripe and it's fairly straight and even. If there is any doubt then I would suggest going to another store. Also if you have a non-chain store (a.k.a. Mom and Pop store), they will sometimes know more about their fish (especially if they are ONLY a fish store) and have them properly marked in the tanks.