I think the best way to know if your workers are happy working for you is just to ask them. It's not always easy to do, but it's important. In my experience, it's not enough to just ask them once in a while—you need to make it part of your company culture. You can't just ask people if they're satisfied with their job once every six months and hope that they'll give you honest answers. You need to make sure that everyone knows that their opinion matters, and that they have a safe space where they can express themselves without fear of repercussion.
I also think that asking questions like "How do you feel about this?" or "What would make it better?" will help you get more useful information out of employees than simply asking them if they're happy.
I also think that asking questions like "How do you feel about this?" or "What would make it better?" will help you get more useful information out of employees than simply asking them if they're happy.