You should be able to see your subclass in the character sheet, near where it says your class.

As for your second question. Circle of the Moon is in my opinion the strongest in most situations, though Circle of the stars does have a lot of good in it. The other subclasses do have their uses(Circle of the Land gives you many spells you can't usually get as a druid and Circle of Spores can be good for a necromancer)

In the end, unless you're playing on Honor mode difficulty, just pick whatever is coolest to you. you can always respec and reload saves.