The absolute giveaway always is the material. Fakes by their nature are cheaply made and of a cheap often coarse material. If you you do not have a feel for designer materials, go into your local designer store in the nearest big town, preferably one that stocks a number of brands and just feel the shirts, trousers, jackets etc and notice the fine quality of the material. For some time I kept two Thomas Burberry shirts to show people the difference between fakes and the real item. They looked absolutely identical even down to the sown on Burberry logo. It was only when you felt the fake material that you realised it was coarse and fairly uncomfortable to wear, whereas the real item was of a fine and comfortable material.
A second giveaway is the stitching Look at sown on labels, especially on Jeans, trousers and jackets. If you can see the stitching through the other side without having to look very carefully, it is almost certainly a fake. You may be able to see the stitching of the real item if you look very carefully, but it will be fine stitching and of a cotton that blends in very well with the main material. You won't for instance see thick white cotton on a beige jacket, which I saw once on a CP Company fake. Also there are quite often crudely stictched on fake labels that aren't straight or in the right place.
A third giveaway is the buttons. On better fakes the buttons are also properly faked, but some fakes are so cheap they even have the wrong buttons! So if you buy a Versace shirt and the buttons say HK368, you've been had.
Finally a few guidelines for avoiding fakes in the first place.
1) You are much safer buying smaller designer labels than the big international designer names. Go to India or the Far East and you will see market stalls awash with designer fakes of everyting. They are always the big names people recognise. Versace, Ralph Lauren, Armani, Lacoste, Stone Island, Diesel, Prada to mention only a few. In menswear, labels like Nicole Farhi, Ozwald Boateng, John Smedley, Kenzo, Canali, Zegna, Ted Baker, Nigel Hall again to mention only a few are rarely faked.
2) Beware the acronym BNWT (Brand New with Tags) on the big designer names. Unless they are at a sensible price, they will nearly always be fakes. Once again if you ever get the chance to visit India or the Far East, you will realise just how many of these are produced.
3) Don't accept the sellers description. If they are dishonest enough to knowingly sell fakes, they will likely be dishonest enough to lie in the description. Comments like 100% genuine are not worth anything as dishonest as well as honest sellers are just as likely to make that claim. Also comments like 'Bought last year from Selfridges' can be made by honest and dishonest sellers alike. You can get a feel for an honest seller, but I for one don't always get that right.
4) Check the feedback. If there are a number of feedbacks suggesting the items are fake and the seller is continually selling the same sort of merchamdise, be very wary.