Knowing how to play with what you have seen is key as well.
If you are front loading and you know the dealer doesn't have a X under you still have an advantage but the play is different if you know for sure what the dealer's card is. You have an idea of what it is ie I know it is an 8,9,10 or I know it is a 2 or 3. You need to know how to play all of this. This is akin to memorizing several different BS charts.
This is the easy part, seeing the card is 99.99% of the challenge.