Basically the same way high volume speed traps can give speeding tickets. You have an observer or 2 watching the gates, CCTV might be sufficient, with a real-time feed of taps and notify officers further in the station with a description of the person and which way they went (eastbound escalator for example).
The observers can:
- Watch for the umbrella/bag/etc gate opening trick, and
- Watch for special fare (child) taps where the person may not fit that category
- Watch for people passing through crash gates or fare booth without paying
Station CCTV would typically be sufficient evidence for the courts to know the person receiving the ticket was also the one committing fare fraud 1 minute earlier.
Perhaps add 1 plain clothes near the entrance for those who try to run from platform level officers.