When you buy them look for otos with a rounded belly. If you can't see that ask when they are feeding their fish and return then to see which fish eat and make sure you get them or at least one of them that eats.
Otos are strange. But if you have one older oto in your tank and you get new ones they tend to do better then if you introduce a group of new otos on their own. It seems that the resident oto tells the others what is good to eat. If there is not enough algae in the tank (the natural food for them) then it may take a resident otos to tell them zucchini is ok.
2x10 gal L174s, 10 gal OEBT shrimp, 10 gal blue bee shrimp, 10 gal Mystery Snails, 10 gal black darter tetras/CPDs, 15 gal Mystery snails, 15 gal L134s/ADFs/clown pleco, 20 gal L46s/L174s/hastatus corys, 25 gal L134s/CPDs/otos/bamboo shrimp, 29 gal Weitzmani and panda corys/zebra otos, 46 gal CAE/L134s/glolight tetras/salt and pepper corys/BNs, rubberlip pleco, 55 gal SAEs/L201s/L199s/BNs/kissing gourami, 75 gal L316s/BNs/mix of old fish folk