vete tú/vaya usted a saber it's anyone's guess ¡vete tú a saber si es cierto! your guess is as good as mine! (al menos) que yo sepa as far as I know para que lo sepas for your information ¡pues no sé qué te diga! I wouldn't be so sure! tener (un) no sé...
Como sabeusted, señora diputada, compete a la Comisión velar por que se respeten las disposiciones establecidas. As you know, Mrs McIntosh, it is the Commission's task to ensure that the standards laid down here are complied with.