in reply to Re^2: if ($?variable_name)
in thread if ($?variable_name)

Seems you have the answer. The tcsh documentation says: 
   $?name
   ${?name}
               Substitutes the string `1' if name is set, `0' if it is not.
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

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