That's almost impossible unless its really obvious withs its tacky design.

If you mean knockoffs, design element will be inferior. But for one to be able to do that needs quite the training of the eye for design.

But often its hard to tell by glance.
One good indication is the person.
Look at the other clothes and accessories he/she has. Are they really different?
Maybe one in a million will have $2 shirt and carries authentic LV bag.
think: is he/she like to invest money on a real one? If it's no, then its fake. 8/10 you'll be right.