Find the parent category of a post

How can I find the category parent for a post. Say that I have:

Category 1
– Category 1a
Category 2
– Category 2a

If i put a post under Category 1a, how can I find out that the parent category is Category 1? With how the taxonomy system is setup, I don’t see a very clear cut way to do this.

Thanks in advance for any help.

** Edit: Sorry, forgot to change the section to post this in. **