I'm assuming you are referring to shuffle tracking, but my understanding is this has not been reliably demonstrated to be effective (capable of producing +EV). Is there some other method you had in mind? You've got me curious...

Edit: the other well-known method is called "hole carding", which means looking for a "front loading" dealer (dealer places the hole card under the top card from the front) who lifts the card too high. Even if you are only able to determine if the hole card is pipped or painted you can get an edge. In practice, casino procedures have pretty much eliminated front loading. In a game that is dealt hand-held (as opposed to shoe), the hole card is supposed to be loaded from the side. In a shoe game, of course, the hole card is never lifted above the felt. So, I guess this is also theoretically a way to get an edge in blackjack, but my understanding is that front-loading has gone the way of the dinosaurs.

Other methods that involve marking or modifying the cards, using dyes or by bending or crimping are all illegal. Nevada is not nice to cheaters.