Goldfish can be difficult to sex even in ideal conditions. Most lfs
only stock juvenile fish that have yet to reach sex maturity so looking for breeding tubercles in male fish will not possible. Venting a fish will also be difficult as few (if any) stores will allow you to pick up and visually inspect a fish. I would venture to guess that atleast 99% of any lfs
have zero clue if their fish are male or female. Theres also a question of 'true' identity of male vs female in any commercial goldfish (vs private breeder) as hermaphrodism is more common due to hormone use. Ill post a link below with more info on sexing goldfish.
If you want to stock specific sexes, you will need to purchase from a private breeder that can sex the fish for you.