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.
20g L174/L46/RCS/hastatus/pygmy corys, 10g OEBTs, 10g blue bees, 10g CRS, 15g L316/L46/Amano shrimp, 10g black darter tetras/CPDs/micro rasboras,15g ADFs/clown plecos, 25g L134s/CPDs/otos, 29g Weitzmani/panda corys/zebra otos, 46g CAE/L134s/CPDs/salt and pepper corys/BNs, rubberlip pleco, 55g SAEs/L201s/L199s/BNs/kissing gourami, 75g L316s/BNs/mix of old fish folk, 30g L340s/L211/tangerine tiger shrimp, 10g tangerine tiger shrimp, 20g L134/habrosus corys/RCS