A Solution for sniffing

[fadi]

Hello Folks,
I think i am being sniffed by somone on my network, and i was wondering. is there an application to check wether i am being sniffed or not, and if i was, how can i fix that ?(like PGP for mail, what about other protocols)
P.S. : Running Linux Slackware 8.1 (if that would help)
cheers,
Fadi R. Khouja