Originally Posted by
the810
Where did you buy the ticket? If it's an ÖBB issued ticket, it should say "schlafwagen" followed by the type of the cabin somewhere on the ticket.
Edit: Or go into "service details" on your booking and it should be shown there too:

here's what it shows:
Sleeper cabin - Sparschiene Comfort incl. Reservation
Wagon number: XXX, Seat number: XX, XX