Make a handwritten card with the dates and mint marks, variations and errors you are looking for, or use the notepad function on your phone to create a reference that you can quickly refer to.
Over time you will memorize most of the rare ones and will be able to distinguish them by site.
But until then the reference will help you catch rare coins, and can serve as a backup if you are not sure.
New rarities are discovered and minted often. So keep your list updated.