Is it possible to determine the class of a Gas Giant without scanning it?
From the same wiki:
Class II gas giants have primarily hydrogen and helium atmospheres. Water vapour in the upper cloud layers gives them a much higher albedo. Their surface temperature is typically up to or around 250 K.
So it looks like you can't really tell. Class is based on the composition of the planets, not mass, size, period or distance from star. Sorry.
You can tell what class they are if you familiarize yourself with what the hologram looks like in the HUD.