Apple told me the way to know if the program is legitimate is to only download through the App Store (or iTunes). If the program isn't available through the App Store, it can be fraught with all kinds of viruses and trojans. All software through the App Store is heavily screened for such, and even the updates. It's a very secure way to get software, which is why Macs do not get viruses easily. You'll probably never get a virus if you don't download through 3rd party vendors. It's one of the many benefits of buying a Mac.