the thing i would do is take pictures with you of genuine products, or make notes on what features the genuine products have. it would be a good idea to have at least images of the logos with you.
for example if you wanted a touche eclat concealer, find one over in your country at the YSL counter and make a note of EVERY feature on that thing, inc the box it comes in.
getting to know what the actual product looks like would really be the only sure fire way i can think of to spotting fakes.
some common fake mistakes though
-logos that read right to left downwards or are mirrored.. this is easy to do with the YSL logo as all the letters are jumbly looking in the original logo anyway- or they may just say YSL and not yves saint laurent...
-incorrect measurements or sizes on the boxes, i.e. should read 20ml for a touche eclat but actually says 10ml
-wrong colours- memorise the shades of the products you're interested in. eg there isn't a touche eclat in "caramel"
-always check the box- the box may appear genuine (and it might be) but whats inside it probably isn't.
as for dior.. i found this guide for you
eBay.co.uk Guides - How to spot Fake Dior Lipstick
hth!!