A casino uses particular software to run the games you play, and they either have to make the software themselves or buy (specifically “lease”) the software from someone else. If the casino built their own software, then they can do whatever they want with it- couple this with a lack of proper regulation and you have some warning flags going up.
This is not to say that some casinos with their own software are not fair and honest- like 888 and Party Gaming- but it is something to consider. Other groups license their software from a company, like Wagerlogic, and if you are familiar with and comfortable with Wagerlogic software you’ve played at one casino, you can feel pretty good about playing it at another.
When it comes to “who is watching” the casinos to make sure they aren’t cheating you with their games, look for something like the eCOGRA ("eCommerce Online Gaming regulation and Assurance") seal of approval. This outfit is a third party watchdog, although originally created by one of those software manufacturers (Microgaming) it does monitor and rate other casinos as well.