Is there an easy way I can tell if a MTG card is "rare" just by looking at it?
Yes, cards have a rarity indicator (at least in recent years). Each Magic card has an icon below and to the right of the picture called the expansion symbol, which indicates the expansion set that the card is from. The icon's color indicates the rarity of the card, with the following key:


Black: Common
Silver: Uncommon
Gold: Rare
Red: Mythic Rare


To get an idea of the relative rarity of these cards, we can look at a booster pack. Recent booster packs each contain about 11 commons, 3 uncommons, and 1 rare, with about a 1 in 8 chance of the rare being replaced with a mythic rare.

A couple of things to remember: very old cards, like from Alpha and Beta, don't have expansion symbols at all. Other old (but not as old) cards have expansion symbols, but they're all black. And some cards have been printed in different sets, and have different rarities in those sets, so the icon on the card may not match the rarity in the most current set.