Some teachers don't like beginning language students to buy Spanish-English/English-Spanish (or whatever) dictionaries, but I think they're very valuable tools. But what they say about books in general holds true for good foreign language dictionaries: you can't tell them by their covers. Some dictionaries are great, while others are terrible. But there are a couple of things to look for to see if the dictionary is worth buying.
First of all, check the publication date. All languages change, and so do the words that they contain. Not only do new words show up in new dictionaries every year, but old words may go through important changes over the years, as well. For example, today words like "web," "pad," and "tablet" have definitions that they simply didn't have a decade or so ago. In general, you will want to buy a more recently published dictionary if you're a beginner.
Second, in the English section go to the word "date." This is a word that has lots of different meanings: the date on a calendar, the fruit of certain palm trees, to go out with a boy- or girlfriend, to tell the age of something, and (in expressions like "to date") the present day. A good dictionary will tell you what the different translations of "date" are used for:
date - n. (botany, fruit) dátil; (botany, tree) palmera, datilera; (calendar) fecha; (outing) cita; (boy or girlfriend) pareja; v. (to go out with) salir con; (letter, ticket) fechar
and so on. A bad dictionary will simply give you a list of words, with no clues as to how to use them: date - dátil; fecha; cita; pareja; fechar. Language teachers can always tell when students have written something using a bad dictionary or didn't pay attention to the usage indications in a good one - they turn in the equivalent of sentences like "I captured the bus" instead of "I caught the bus."
Third, in the Spanish section go to the word "torta." Depending upon the context (including the country where it's being used) "torta" can mean a cake, a pie, a kind of sandwich, an omelette, or a slap in the face! A good dictionary will not only give you several different different definitions, but will tell you where they are used:
torta - n. slap, punch; (food) cake; a type of sandwich (Mexico);
idioms: _____ de huevos - omelette (Mexico)
agarrar una _____ - to get very drunk (Spain, informal)
And once more, a bad dictionary will give you just a bare list of words, with no clue as to usage. (When looking for a Portuguese-English dictionary, take a look at the entry for "bicha." In Portugal it means a queue or waiting line; in Brazil it's either an insulting word you shouldn't use, or a case of parasitic roundworm!)
Taking a couple of minutes to take a look at these three things will give you a good indication of whether a dictionary is worth buying, or whether you should put it back on the shelf and keep moving. It can mean the difference in your language use, because you'll be learning the correct translations of words the first time around - and it might make a difference in the grade you get.