gTalk status icon on the notification bar
Provided you have blue bars for your wireless and/or wifi connections and have logged into GTalk you should always be connected.

You have to sign out of GTalk or lose connection with the Google Servers (gray signal bars) in order to be offline.

Short story: as long as you signed in and have a data connection you are online for GTalk.

If that doesn't give you a warm fuzzy feeling then use a third party app like Trillian and use it for GTalk. Trillian might be able give you a notification icon.