Unfortunately there are a certain amount of people who are going to scam you. In the case of workers comp, your employees may find doctors or other 'experts' who are willing to jump on that train for the money. It's maddening, but impossible to mitigate against entirely. No matter what kind of business you operate from the cushiest office job to the most hazardous conditions, make sure you understand the basic practices which can help reduce or eliminate the number of incidents you may face.
Use Your Network
This is probably going to be your best way to go about preventing workers comp fraud, but it's also going to be the hardest one to navigate. As a boss, you have a hard job to do when trying to strike relationships with your employees. You've probably seen first-hand how being their friend is definitely not a good strategy. They need to respect you, and if they don't then you open yourself up to a world of problems. However, since you can't spend all of the time with your employees, it's fellow co-workers who can speak up about whether or not they feel that someone isn't trustworthy. If you ask them directly though, they may not feel comfortable telling you (or feel like they're being a snitch.) The best way to go about this is to either find a way for people to be truly anonymous when giving out information or creating an environment where truth and morality are the most important values in the work place. Show your workers how much you need them and they'll be more likely to respond by helping you and the company out. In order for this to work, you'll need to have sound hiring practices, so don't skimp on calling references and former employers before hiring someone. You can go the drug testing/video surveillance/lecture route, but it won't create a healthy environment. People will start their jobs already feeling like criminals, and no one wants that kind of treatment. It will likely just make people resent the company.
Have Strong Policies in Place For Claims
There should be a specific way to handle injury claims should they happen, and it should be made clear that the investigation process will be thorough. Send out the information for people to read or have a meeting about it. If the workers see that it's important to you, and that you're ready for whatever may come your way, they may be less likely to even consider fraud because they know you're on top of it. It also helps you feel prepared if the worst does happen to strike, and can save you from feeling like you're powerless when unexpected events occur.