For older cards, there's no good indicator on the card itself to let you know how rare it is. You've got to know a bit about the cards to know how valuable it is. For newer cards, they indicate on them how rare they are by what color the expansion symbol is (that's the little symbol on the right just below the artwork). Use that as a good guide to determine how valuable a card may be. And, here's my "what to look for list".
1) If there's no expansion symbol, you've got a card from a core set from 5th edition or earlier (that's 1997 release at the latest).
1a) If it has a black border, it's from Alpha or Beta and could be
very valuable. Even if it's not a rare card from those sets, they often times sell for a decent amount of money due to their age.
1b) If it has a white border, it's from Unlimited, Revised, 4th, or 5th. Some of these cards are very valuable, but plenty of them are garbage. You'll have to do some research to determine what is what.
2) If there is an expansion symbol, check it.
2a) If it is a sword, an anvil, the top of a column, a crescent moon, a crown, a globe like thing, a snowflake, a banner, a palm tree, a V inside a triangle, a book, a storm-cloud, or a portcullis; there is no additional information about rarity available (these are all released Feb 1998 or earlier). The earlier in my list the symbol is, the older the card is. Every card (with 1 or 2 minor exceptions) from the Crown set (Fallen Kingdoms) and the Globe set (Homelands) is pretty much garbage. The "expensive" cards from those sets are only worth a few dollars. Every other expansion has cards worth real money.
2b) If it's any other symbol, proceed to 3.
3) Check the symbols color.
3a) If it's black, it's a common card and not worth much at all (maybe $0.25 if you're lucky). (Keep in mind that all the symbols from 2a will always be black, regardless of rarity).
3b) If it's silver, it's an uncommon card. Some of these are worth a few dollars, but most won't be worth much.
3c) If it's gold, it's a rare card. Not all of these are valuable, but many are.
3d) If it's purple, it's a "Timeshifted" card from the Time Spiral expansion. These are reprints of old cards and some are very valuable.
3e) If it's red, it's a Mythic Rare. These were only introduced in the last few years and are even rarer than rares. Some very valuable.
4) Anything that is a "foil" card gains quite a bit of value. Even a common foil card can be worth a few dollars. (Foil cards are the shiny ones).
5) Being signed by the artist is going to add some value, but it depends on the card, the artist, etc.
If you're really looking for a jackpot, keep your eyes out for one of the "Power Nine". They are: Black Lotus, Mox Sapphire, Mox Ruby, Mox Emerald, Mox Pearl, Mox Jet, Time Walk, Timetwister, Ancestral Recall. Library of Alexandria is often times thrown in when talking about the "Power Ten". All of these cards worth are dependant on the card and which set it's from. But, they are easily worth $100+ to $15000 for an alpha edition Black Lotus.
There's also some
incredibly rare cards that are worth small fortunes, but you're unlikely to stumble across them. Promotional cards (ones that were made in limited quantity specifically for special events), Summer of Magic cards (a set that was made without all the "Demonic" references and was later destroyed but some cards accidentally were released), and other "Special" cards (for example cards given to World Champions as a form of trophy, or the one Richard Garfield had made to propose to his wife). The low end of these sell for $100+ and the high end are a question because the cards have never been sold
