Is there any way to detect (from a Linux box) if someone is packet sniffing on a network.

Yes, as long as you can hack together raw packets (or have some tools that can). There are some details in the 'packet sniffing FAQ' - I can't give you a URL as our proxy will not let me see it - it thinks I am hacking ... :-( The essence of it is that you have to send out packets that the machine shouldn't usually be able to see, but because it is running in 'promiscuous' mode, and seeing all packets on the network, it responds. Google should find it for you though. Kevin.