As a general tip, it's easy enough to spot a marked deck that is "plainly" marked, not using "juice" or "shade" methods. The riffle test, a.k.a. "going to the movies," will show off the markings if you look carefully enough, with rare exceptions - it works best if there aren't extraneous markings (like the Bicycle Series 1800 Marked deck) and if the marking system isn't spread across a too-large area of the card back (like the NOC signature series). When riffled, the backs of a standard deck should all appear the same while the backs of a marked deck, each being unique, will flicker like watching a flip book animation or a nickelodeon.
Once you've revealed that it's marked, revealing the marking system is usually easy enough. "Readers" don't require any special effort - they're usually marked in a plainly-readable manner, usually letters and numbers for values and letters or symbols for suits, concealed amongst the details of the back design. When the system is concealed by some form of covert markings that aren't plainly readable, you need to decode the system. The method I used was to first look for suit marks by sorting for suit, then riffle and seeing what sections of the backs only changed every thirteen cards. Once that was solved, then you sort for value and riffle to see what changes every fourth card - it's a little more difficult, but not my much.
This is most effective when dealing with cards that are intended for gambling use - the markings in such cases are often confined to the upper-left corners of the card backs (and turned around in the opposite corner, the lower-right). This makes them easier to spot when held in a player's hand or left on a table in a face-down spread in a manner to make the indices readable - reverse spreads work great for magicians, not so much for card players. There is at least one deck I know of that's specifically designed for use by magicians in spreads whereby the spread is reversed for the magician, normal from the spectator's perspective and the marks are inverted for the spectator and in the wrong corner, making them harder to spot for him/her but plainly readable to the magician - almost completely unusable for card play, due to the marks being on the wrong corners, but because of that, slightly harder to spot. (Not too much harder, because they are readers and not encoded in any way...)
