air con refrigerant leak, how to locate leak?

[omodos]

hi, ok temps have soared here and so i tried the air con , alas no joy, just 'normal air' pumping in cabin, vehicle is a 2004 x5 3.0d, took it to a local air con person and they put it on the machine and told me it was all out of refrigerant, so they pumped some in an told me if there is a major leak our machine will show drop in pressure, if not and you find it empties out soon then you need to go get it checked elsewhere....hmmmm cost me 40 euros and to be honest cabin was freezing much better than i have ever felt before, but anyone any ideas?and does the system need to me 'flushed' every 2 years as one of the young lads told me?and the leak any easy way of finding it? thanks,

ps good luck to the u.s soccer team