Mark Driscoll has a nice step by step approach to determining whether you are living in legalism or not:

1. Make rules outside the Bible

2. Push yourself to try and keep your rules

3. Castigate yourself when you don't keep your rules

4. Become proud when you don't keep your rules

5. Appoint yourself as judge over other people

6. Get angry with people who break your rules or have different rules

7. "Beat" the loser

You can read the rest here.