You can easily find if your friend is invisible or actually offline on GTalk using this trick. But to use this trick, you have to catch him online at least once. So if you doubt that one of your friends is invisible from you too often, then –
1. Whenever you see him online, just set the chat as off the record with him.
If you are using the Gmail’s inbuilt chat facility, then simply click on the Video & more link in the lower left corner of the chat widget and click on Go off the record.
The message You are now off the record will be displayed after that.
If you are on Gtalk, then click on settings > chat, and check the box that says, don’t save my chat history in my gmail account. And save this setting.
So, now your chats with your friend are off the record.
2. Now you can easily detect if your friend is actually offline or just invisible just by sending her any message. If she is actually offline, then the following message will be displayed,
And if she is online, nothing will be displayed, which means that she actually received your chat message, and thus, is online.
Why it works?
By default, all your chat sessions are stored in your Gmail account (and in the account of the person who chatted with you). Now, once your chat with one of your friend (whom you are spying ;) ) are set off the record, they are no longer stored in the Gmail account. So if you send a chat message to her, it must be displayed as a pop up windows. If she is actually offline, it can’t be done, so the message xyz did not receive your chat is displayed. On the other hand if she is just invisible, she can see the pop up, and hence no message is displayed.
Note – I am assuming that your friend is using the GTalk gadget integrated in Gmail, since the standalone Gtalk client do not support invisible mode yet. Although you can simulate the invisible mode on standalone Gtalk client by staying always idle.