
Canonically, even without his ring, Sinestro's understanding and discernment of individuals' primal fears has been noted (presumably due to his close relationship with the fear entity Parallax and a natural inclination to acute observation). This discernment also extends to individuals capable of causing great fear in others.
Upon his arrival in the City, the Porter will be boosting this ability so that he will be able to read an individual's fear or if the individual is capable of causing great fear, even over the network. The more strongly fear emanates off of a person, the easier it will be to read. If an individual tends to bury their fears, he may only catch a glimmer, or if they are truly fearless, he will read nothing at all (but note that this is rare--even Green Lanterns, who are selected for their ability to overcome great fear, are susceptible to fear). He will not be able to do this if the person is communicating solely via text (voice or video is necessary), unless the communication is obviously frightened/threatening/etc and no one would need to channel a power to know that it is fear related. He will also not be able to do this without your ooc permission!