How to Count Cards
Card counting doesn't require advanced memory techniques. See the tips below for more information on how to do it.
How to Count Cards Video
Here's a video on how to count cards:
How To Tell If You're Playing Blackjack With A Card Counter
An article in Forbes describes what to look for:
In blackjack, if you count cards, it's very easy to tell if other players are counting. Counting cards changes your play style in certain ways. You assign each card a value when counting and then add to the running "count" when cards are played. You will tend to bet high when the "count" is high, and bet lower when the "count" is lower. Even with different counting systems, counting always points in the same direction. If someone is betting high when the count is high and lowering the bet when it drops, then perhaps he or she is counting. If the person is sitting out when you are sitting out, then the person might be a counter as well. To a counter there are some very clear signs that people are counting. This is why many casinos hire former counters to detect and confirm counters.
How to Count Cards Tutorial
Here's a tutorial on how to count cards.
There are several methods, which you can find on the Wikipedia page.