The foolproof way for telling apart regular people and teekkaris is from the teekkari cap: a white cap with a black tassel. The cap creates social cohesion between teekkaris and you can even roughly estimate how long ago a person earned theirs.
Teekkari caps can not be bought from a store and as hinted earlier, each freshman has to earn the right to wear it by completing their freshman year in an honorable fashion. Active freshmen earn their right for the cap on May Day’s Eve (April 30th). The right to wear a teekkaricap can also be earned by graduating.
Each guild places an order for teekkari caps once a year from January to March at the latest. The caps are manufactured by hand in Helsinki by E.R. Wahlman. The cap has to be worn paired with the emblem of the University of Turku: a winged, flaming torch. The cost for these is roughly 135 euros and it goes straight to E.R. Wahlman and TYY.
The cap is worn between May 1st and September 30th. In order to wear the cap outside these dates, a teekkari cap special use application should be sent to the board of Teekkari Commission which they will either endorse or reject.
You can read more about the technicalities here (currently only available in Finnish)