1. You have both defined your relationship
You can't say you are in a mature relationship when you are yet to define if you are in a relationship with this person or not.

2. You both know each other's point of view on some important issues like marriage and children
Sometimes you have to really have to see each other's opinion on important issues like children, cheating, marriage. These questions would come naturally, all you need is just clarity.

3. You don't get pissed when someone hits on your partner
As long as you both make it clear that you are both off the market, there's not need flying off the handle when someone hits on your partner.

4. You aren't always together
They do stuffs apart and aren't always glued to each other. It doesn't mean they are growing apart, it means they are building a life for themselves.

5. There's no worry about your relationship crashing
You aren't threatened in the least bit about your partner suddenly ending the relationship.

6. You brag to others about each other's accomplishments
In a mature relationship, you want the other to be fulfilled at work and have no issue telling your friends about the tiniest things they've accomplished. It means you are both rooting for each other's successes.

7. It's easy to be with each other
Relationships built on drama isn't a mature one. Showing someone you love them and getting back that love in return is a sign of being in an adult relationship.