Mac chat/instant-message programs like Adium, Jabber, and AIM require you to enter your chat service account login and password information in the preferences, to enable them to function. How can I be reasonably sure that entering this information in the app(s) is secure? I am concerned that the apps may be forwarding my account credentials to unknown or malicious locations.
As a rule, unless you've reviewed the full source code of an application yourself, you have to trust the developer(s) of the application that they're not going to misuse your login credentials.
Depending on your level of concern, you might set up a separate, dedicated Gmail account / login, exclusively for use with GTalk and related services.
I'm uncertain how Adium's network protocol operates, but if it uses well-known servers and ports, you could also use LittleSnitch to inspect and approve outgoing network connections from Adium, to be sure that it is communicating only with places it's supposed to communicate with, i.e. official GTalk servers. (LittleSnitch inspection is only feasible with well-known servers, not with peer-to-peer based services.)