I could tell you the answer, but learning it would be better.
If you have the tools to try it:
2 or 3 computers with wifi
1 wifi router.
Use airodump do capture the data. You can have the data going between boxes, or out to the internet. Then try decrypting the data with your known psk, and use wireshark to look at the resulting file.
You can find how to do most of that looking at the aircrack-ng website. You'll want airodump-ng, and airdecap-ng. Both are part of the aircrack-ng suite.