How do you know if your belly piercing is healed? I haven't been getting crust around it or anything and I have been cleaning it and it seems OK

If all goes well, your belly button should be fully healed 12 months after getting the piercing. When I say if all goes well, I mean with minimal irritation and infection. Irritation and infection prolong the healing process. If you want to know when you can change the jewelry, do not change it for 6 weeks, even if it seems fine to you. After 6 weeks, you can normally change the jewelry, but keep the number of times you take it out and put it back in at a minimum. You don't want to cause irritation. :)

If it's a little red or kind of purple does that mean its irritated or infected?

A little red - usually just irritation.

Purple - You need to see a doctor, it is probably infected!