I am mostly using Debian Testing and had never tried using Stable + Backports.

Was reading about Backports and wondering how to know if, the packages we wanted to install is in Backports or not.

For example, after adding backports to source.list, if I wanted to install smplayer, will the results of apt-cache or aptitude search tell me that new packages from backport is available, instead of installing from stable?

Yes, you will see both the stable version and the backports version. By default the stable versions takes precedence over the backports ones so that everything doesn't get automatically upgraded. The backports have the string bpo in the version name, which helps to identify them.

To install the backports version, you can use the '-t' flag to apt-get to specify lenny-backports.

Backports are useful if you want mostly stable and just a few packages from the backports repository.

Not all packages are in backports (you can use the search on the backports site to check). The smplayer package isn't.

I do not think there are any Squeeze backports yet.

Yes apt-cache search will show backports if there is a a version of that particular app in backports.

I use surfraw to search so I type:
debpackages iceweasel
and I get a new tab in Firefox that shows:

Got that, thanks for all the clarification.