One of the best ways to stop skimming is to keep point-of-sale machines out of the reach of the public until a sale has begun. This will not give the scammer time to attach a device to the machine. If your point-of-sale machine must be left out, regular checks of the devices and training your staff on what to look for will give you and your team more knowledge of how to prevent this from happening.
If your point-of-sale machine is mobile, keeping a record of who has the device and where it is will allow you to track down where and when a scam may have happened.
Lastly, a robust security system, including cameras, will watch over the device to observe if anyone happens to tamper with it.