I am constructing a likert scale. How could I know about its validity

From here:
"In principle, it's a good idea to run the test on a group whose attitudes are known. For example, you would expect British National Party members to score negatively on attitudes to ethnic minorities. If they don't, there's something wrong with your survey. If they do, then your test is valid. In practice, though, it's not always easily possible to find such a group."

And here's a more in-depth article you should find helpful:
http://jonjayray.tripod.com/consval.html

I'm doing a demographic survey in my city .which of the sampling technique should I use?