How can I tell if someone is online and available to talk?

"Green means 'go'!" All of your friends that are available to talk will appear at the top of your Friends list, with a green ball next to each of their names.

A red ball next to someone's name lets you know that this person has designated himself/herself as 'Busy.'

If one of your friends is away or inactive, an amber ball will appear next to his or her name.

And when your friends are offline, a grey ball will appear next to their names to let you know that they are not signed in to Google Talk.

No ball next to someone's name? Invite your friend to Google Talk.