Best way to find long trail keyowrds?

I have been looking into his for some time now, but I just can't find a way.

I use google keyword tool, so lets say I type in "sports car" as my main keyword.

Most of the results will be like:

"buy sports car"
"red sports car"

etc, but none of them will be like:

"how to install a radio in a sports car"

Which are long trail keywords I am after.

Do you have to guess these? Is there some way to do it otherwise?

Thanks

There was a tool called SEO hades keyword zombie that did this pretty well. I don't have the link by try doing some searches here and you should be able to find it.

Do you need the longtail keywords to have any searches? If you don't, why not?

There are a bunch of WSO's that sell these kind-of tools that get keywords from Google Auto-suggest.