Real-time brain scans coupled with a machine-learning algorithm can reveal whether a person has memory of a particular subject, but with a little bit of concentration people can easily hide their memories from the computer.
Memory is obviously important, in areas like eyewitness testimony, medicine and even marketing. Programs that can read a person's brain scan data and surmise whether that person is experiencing a memory could be important for those reasons.
But with just a little bit of coaching and concentration, subjects are easily able to obscure real memories, or even create fibs that look like real memories, on brain scans. For cooperative subjects, things are good, but for high-stakes situations knowing they can be spoofed is important.