Go a few times, every store has bad times and you don't want to miss out on an otherwise good shop because they just got in an order that wasn't good for some reason (bad shipment from the wholesaler, delayed during shipping, someone called in sick so they are running behind, etc.).
Don't only go on Saturday, try to go during the week, they will be slower and able to spend more time with you. This will allow you to get a better idea of their knowledge. Ask a few questions, some you know and some you don't know.
Ask about an item and if they can do anything about the price. In today's world this can go two ways 'no, the internet is killing us and if all you care about is price then you are putting us out of business so thanks for nothing, we can't compete with online pricing', or 'we can do something about it but we may not be able to match the price, at least not on everything'. IMO
it is not bad if they don't want to work with you, I tend to agree with the first statement, for the sake of business I have to go by the second.
Ask about a special order fish, something rare and harder to come by. Good shops should take your name and number and/or email and try to get it in for you.