Paul is probably referring to the spiritual gift of distinguishing whether what people say comes from the Holy Spirit or some other spirit (1 John 4:1) or to the differentiating (i.e., evaluating their truthfulness) of the words of the prophets in 1 Corinthians 14:29. While the gift may still exist today, the measure of what is true is Scripture. Therefore, this gift must be practiced in accordance with what is true in Scripture.
See the study entitled, The Argument of 1 Corinthians 12-14 on our web site in order to understand the overall movement of this portion of Scripture. Then read these verses in that context. See also the articles in the Prof’s Soapbox on these issues. Also, I recommend that people read 1 Corinthians as a whole several times, and then, chapters 12-14 (which are a unit and together constitute an answer to the Corinthian’s questions about the spiritual gifts and the so-called supremacy of tongues).