Card counting it doesn’t take a lot of concentration or mathematical calculations, and demands little use of your memory. You already know that a deck rich in face cards is an advantage to the player. All you need to do is casually observe the flow of cards coming out of a deck or shoe. 
The key to this system is to look for extremes. The absence or predominance of exposed high cards is easily noticeable by casual observation.
Often you will play several hands where the mix of face cards and low value cards are relatively even. Casual card counting is something you should always do unless you are following a strict card counting system such as the Hi-Low system. It won’t guarantee winners for you on every hand, but it will give you a slight advantage.