You won’t know you’re looking at a ghost fare until you’ve selected your flight (or flights) from the search results. A few things will tip you off that what you’re seeing is likely a ghost fare.
First, the flight will either suddenly be much more expensive, or you will be instructed to “call airline” to book (if you call, you’ll be told the fare is no longer available). Additionally, the other fares that are bookable online will typically be priced significantly higher.