bargaining
Hat Yai is a touristy area with a flood of tourists from Malaysia and transit passengers. Local people seem to develop a tradition to overcharge all foreigners. The situation is one of the worst even compared to other touristy areas in Thailand.
Learning a few Thai phrases usually doesn't keep you from being overcharged because Thais get used to Malaysians pretending to be a local by speaking a few Thai words.
Talking about honesty doesn't mean much to the people in Hat Yai, but smiling will help. When you are asked to pay overcharged price, name any poor country, say you come from there, and insist that you don't have money. Sometimes it works.
See how much local people pay for their food. It's unwise to ask them directly because they don't feel like "betraying" their neighbours by telling you the real price.
Use common sense. Street vendors may charge you 25 bahts for a BBQ stick of meat balls local price ~ 10-12 bahts. This is almost the same price as in Singapore, so you better walk away.
Consult the staff in your guesthouse and ask them how much they pay for a particular food such as noodles and congee.
