I get emails from Disney Destinations: your monthly "Hello" email and then on occasion pins. A pin email will say something like "Enjoy a great dining deal ___ family", "Save on a Disney Resort stay, ___ family", "Monster savings on a Disney Resort stay" etc. So just by the subject of the email you'll know if it's a pin or not. When you receive a pin email, read through it and then at or towards the bottom of the email where it gives you the phone number to call (or the book now link) it will then say "Be sure to have your PIN ready" and then it lists your pin under that.
Hope this helps!