For the most part, alpha, beta, and unlimited series (the first years, 1993, 1994) cards can be worth substantial money - more than $50. I'm sure there are exceptions, but with over 20,000 different cards, I'll let the MTG folks flame me over this broad statement.
You can tell the rarity of most of the cards by the color (or lack thereof) of the symbol that is halfway down on the right hand side of the card. If there is no symbol, it is an earlier one, you probably ought to look it up.
Most of the common and uncommon cards have values from about 10 cents to 50 cents, if you go by the rarity guide. Some will go for 1-2 dollars. The rarest card, black lotus, regularly sells for over $1500, with one instance of a perfect (10/10) graded card selling for $20,000.
Here is a good guide to help you identify the card series.
Here is a rarity guide.
Here is my auction for 5,200 common MTG cards, ends May 8
Update: note from buyer:
Sorry I haven't gotten back with you sooner. This was the coolest box of cards I have ever gotten on ebay. I am still going through them. Please let me know if you ever have another lot like this. TJ
So in retrospect, I probably should have just counted the cards and sold them. I probably could have got a lot more money for the lot from just advertising a completely unsorted lot of 6000 cards, nad it would have been a lot faster.