Функция греха не дает правильного ответа

if ( typeof variableName !== 'undefined' && variableName )
//// could throw an error if var doesnt exist at all

if ( window.variableName )
//// could be true if var == 0

////further on it depends on what is stored into that var
// if you expect an object to be stored in that var maybe
if ( !!window.variableName )
//could be the right way

best way => see what works for your case