Checking if a variable is defined in SASS

AS the title says I am trying to check whether a variable is defined in SASS. (I am using compass if that makes any different difference)

I've found the Ruby equivalent which is:

defined? foo

Gave that a shot in the dark but it just gave me the error:

defined": expected "{", was "?

I've found a work around (which is obviously just to define the variable in all cases, which in this case it actually makes more sense) but I'd really like to know if this is possible for the future