I'll give the short version answer here. The long version can be found from an article like this one.

For the English Pokemon cards (physical), look at the bottom left of the card. If you see double stars then that card is an Ultra Rare card. Circle is common card (easy to find and usually low price), Uncommon is a diamond and a single star is a rare card.

A card with fancy artwork would either have one yellow star on the bottom left or two yellow stars if it is a nicer art with a higher rarity.