How can I know if anyone in the network is monitoring my chatting on yahoo/msn

If you have a good sys admin you'll NEVER know!!
There's some many network monitoring progrmas out there you don't have a hope !

You can quite often use tools that monitor traffic flow on your PC.
If you are doing absolutely nothing on your PC, and things are still flowing, then more than likely someone's watching you.

Unless you are good with dataflow, don't even try decrypting the packets :)

Basically, as Ben stated, you will NEVER know...