We need more information. Do you want to 'detect' the voltage or do you want to 'measure' the voltage? By detection we only want to know if the voltage is above a certain point - a comparator is normally used here. If you want to measure the voltage and give a read out in some form then that opens more questions. Do you want to use a microprocessor? Or do you just want a digital volt meter? I would suggest you use Google. Or you could just go to the local Dick Smith store and buy a digital multimeter for $10.........