How do you identify the key pin in a spare situation?

Answer 1

Whatever pin is closer to you, so if you had to 2-9 or something similar, you throw at the left side of the 2, because it is closer. If you leave a spare, or split where the two pins are in the same row, like the 7 ten, you would throw at whatever pin you felt more comfortable throwing at

Answer 2

It's the pin you have to hit in order to make the spare. In almost every case, that would be the pin which is closest to you, the bowler.