Yahoo! Messenger is a popular instant messenger client provided by Yahoo! Inc., the same company that created the famous web search engine. The program can be downloaded free of charge from Yahoo! Messenger. Login requires a free Yahoo! user name which also grants access to further Yahoo! services such as Webmail or Webspace.
The latest version of the Messenger (7.0.2.120, December 2005) comes with many features, most of which were already included in previous versions. For a long time users have been able to hide from others by setting their status to invisible. With version 6 Yahoo! Messenger introduced stealth settings which allow users to target a single contact on their friends list and choose whether they want to appear offline or online to this person.
However, there are several workarounds to find out whether a contact is truly offline, invisible or hiding through stealth settings.

Probably the most popular tool is Buddy Spy. It's a standalone program which can be set up to check on the true status of Yahoo! Messenger users. It will display whether a user is offline or invisible, if they have stealth settings applied or if their webcam is running. The initial configuration is the most tricky part and a Yahoo! ID ("bot") is required. Once everything is properly set up the program is very easy to use.
Buddy Spy can also be utilized to display fake idle or webcam status on the Yahoo! ID serving as a bot. The problem is that if the main Yahoo! ID is used as bot, offline messages or alerts to that ID may be lost. Thus it is recommended to create a separate ID solely for the use of Buddy Spy.

A similar program is Nuotex BuddyCheck. But unlike Buddy Spy it is not freeware. After a ten day trial users need to buy a license to continue using the application. And apparantly it has no advantages compared to Buddy Spy.

There are methods that do not require an additional program. They have some disadvantages, though. One is that the person "spied upon" may realize they are being "exposed" in case they are really online. Another is that these methods don't work with all versions of Yahoo! Messenger or only work for revealing stealth settings.
On ikitek, a site dedicated to these alternative methods, four different strategies are currently described, two of which no longer work as of version 7 of Yahoo! Messenger. One method will detect the stealth settings of a queried user. You simply surf over to the site, click on the respective description (Stealth Settings Check), scroll down to the text field, enter the user ID to be checked and wait for the search result. It will tell you whether the user is online or offline. If, however, a user is set to invisible the result will be "offline". Also, if the user has opted in to not show their status on Yahoo! web sites this method won't work.