How do I inspect Vim variables?

See if this helps: http://learnvimscriptthehardway.stevelosh.com/chapters/19.html. Should give you some insight on how vim variables work, and you can check out chapter 20 as well if you have any difficulties inspecting them due to scope issues.

Like lucapette wrote you can use :echo g:foo to inspect a variable. You can also use :let to see all defined variables and their values.

:echo g:SuperTabDefaultCompletionType

works fine. It gives an error if the variable isn't defined.