Not one line, but possibly easier to read

gistfile1.rb

require 'test/unit'
include Test::Unit::Assertions

instance_true = {:source_dest_check => true}
instance_nil = {:source_dest_check => nil}
instance_false = {:source_dest_check => false}
instance_raise = {}
instance_garbage = {:source_dest_check => 'asdfl;kjag'}

def test(instance)
	source_dest_check = !!instance[:source_dest_check] == instance[:source_dest_check] ? instance[:source_dest_check] : true
	source_dest_check
end

assert test(instance_nil) == true, "Nil was not true"
assert test(instance_true) == true, "True was not true"
assert test(instance_false) == false, "False was not false"
assert test(instance_raise) == true, "Key error was not true"
assert test(instance_garbage) == true, "Garbage was not true"