You'll see there is a flat 'ribbon' cable that runs from the CD board to the main board. You can then count the number of wires in that cable. Or you could just look at the ribbon cable connector on the CD board and look for numbers near it.
My two 21 pin Saturns have a '1' at one end and a '21' at the other.
Be aware though, there is very little choice in modboards. You may be able to get a 21 pin 'pass through' mod (but don't bother asking where) or get a 21 pin 'two wire' mod. You can get the latter from Lik-Sang, Lan-Kwei and others.
20 pin mods are now as rare as... really rare things. If you find you have a 20 pin Saturn, and you want to mod it, it's time to buy a 21 pin Saturn.