joe jobbed or hacked?

[Jon-Paul Kelly]

I just received around 2000 bounce messages from various servers  
rejecting messages (supposedly) coming from my email address.
This has happened to me before but not on this scale. Any ideas on how  
to tell if this is just a joe job or if someone has actually used my  
server as a spam sending platform?
I am on a centos5 box.
Any help on this will be greatly appreciated.