Variable are in scope if they are defined on the page in question or in somewhere in the parent hierarchy of the page in question.

For example, lets say the variable we are interested in is X.

All variables that have been declared using !define can be checked by adding ?variables at the end of the page's url.
For example http://myFitNesseHost/FitNesse.UserGuide.FitNesseWiki?variables will show a table with the names, their values and the pages the values are defined.