It will be something like QO30-40 if it will take tandems. There will also only be certain spaces that will accept them. There will be a slot in the rail in the positions where you can use them.
Take the cover off the panel. On the interior side of the panel tub is a decal saying how many circuits it is rated for. My QO 100A 32-space panel of similar vintage says 54 circuits.
I got burned a couple of years ago doing a kitchen remodel of a house built in 1946. Besides no ground wires to most of the house, the QO panel was full and I needed 4 new circuits. Only the bottom 4 spaces allowed tandems so I thought I was in !
FAIL !
Older QO boxes (1980s ?) can not use the current QO tandems and Square D no longer makes the old ones ! Luckily I got the part number from the guy on the Square D hot line and 5 minutes of Googling I found NOS breakers at a very reasonable price.