I've interviewed quite a few Flash/Flex developers as potential employees for Roundarch. The hard part for me is knowing exactly what questions to ask to be able to gauge a Flash dev's skill level.
So, I made a list. From my experience this list allows me to judge the skill level and experience of the devs I interview.
If you're not quite "Senior" level yet, read through the list and study up from the links. If this stuff is "old hat" to you, please critique this list add more references in the comments.
1. The Elastic Racetrack
You need to understand when events fire, when code is executed, and when the player renders. It’s really the foundational concept of Flash Player.
2. FlexSDK, mxmlc, compc, and all that jazz
You need to know what’s going on when you hit that “run” button in Eclipse.
3. Player Events, Custom Events, Event Bubbling
I can’t imagine where flash player would be without frames and mouse clicks.
4. Statements, Keywords, and Directives
You should never be surprised by a new “word” in as3. If you don’t know what “static” or “override” or others are, learn them.
Someday, you’ll re-read your code and have no idea what it does. The stress here is on proper documenting, but generating asdocs is nice too.
6. Managing visual assets (images, fonts, CSS, etc)
Flash is visual. So you need to know how to manage assets with code.
7. Arrays, Collections, Dictionaries, Mapping
You’re not just working with one MovieClip here, so you need to know how to control multiple objects at once.
8. Programmatic motion (tweening, easing)
If you wanted things to stay still, you should’ve stuck with HTML
9. OOP and Coding Against Frameworks
The days of a one huge .as file are over. You need to smartly move your code into organized and reusable objects. Also, now there are plenty of popular as3 frameworks to help manage your code. Read up on them and start using one.
10. Version control
Unless you think your code is worthless, you need to learn how to back it up properly.