The booking class consists of a letter i.e. C, E, B and T.
When you make a booking you can see your booking class in a box to the right with the title "Earn EuroBonus point".
It is also written on your Electronic Ticket Itinerary and Receipt sent to you by email. There you will find the booking class beneath the flight number.
Note! If you didn't buy your ticket directly from the SAS the itinerary / receipt might look different.