file_exists.rb

module Sass::Script::Functions
  def file_exists(path)
    return bool(false) unless options[:filename] # not all sass has a file name (e.g. std input)
    current_directory = File.dirname(options[:filename]) rescue nil # a sass filename from an importer may not be processable by File.dirname
    return bool(false) unless current_directory && File.exist?(current_directory) # not all sass files are served from the filesystem
    full_path = File.expand_path(path.value, current_directory) # a relative path will be expanded against the current_directory of the sass file, symlinks will be followed, absolute paths will be left alone.
    return bool(File.exist?(full_path))
  end
end

usage.md

Command line invocation with sass

$ sass -r ./file_exists.rb scss/foo.scss css/foo.css

With Compass in the config.rb

require "./file_exists.rb"

In the Sass file

.foo {
  background: if(file-exists("../images/foo.png"), url("/images/foo.png"), url("/images/generic.png"));
}