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How can I know for sure if this is happening?
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You can't know for sure. Any hop from you to the recipient can potentially be sniffed for packets. That's why you should treat IM traffic as any other plain-text traffic: it is not necessarily free from prying eyes.
As was mentioned, if the debian box is a gateway, run rkhunter on it. That will check for promiscuous interfaces.
If you want your communications to be secure, you're going to need to encrypt them, period. That is the reality today.