Like humans, felines blink often to protect their eyes and keep them moisturized. This is done by opening and closing the eyelids quickly, and it's not the kind of blinking we're talking about here.
To identify a slow blink, pay attention to your cat's body language. a real slow blink can only come from a relaxed kitty, so pay attention to whether or not they seem at ease: watch their ears, legs, tail...
Their state of relaxation should also be noticeable in their eyes, meaning that their pupils shouldn't be very dilated.