Students can gather a lot about authors' predilections by what Internet sites they consider valuable and by what news groups they are part of. To use an extreme example, if a student gets involved in an online discussion about gun control and notices that the bottom of the posting that the author also belongs to alt.rec.guns or alt.conspiracy, they'll get a sense of the author's political point of view. But, in general, the conventions that would identify the quality of material on the Internet have not yet been developed. The best indicator so far is the quality of the written texts themselves. If something is well written and makes the kind of sense a good essay does, it is likely to be more reliable, but that can be deceiving.
Images are very deceiving as well.
Some of the most interesting information has no multimedia glitz at all, while some of the cheapest, most dubious information is heavily illustrated. Before the Internet, a teacher who wanted a student to dig into a question would send that student to the library. Kids have to learn how to search without a librarian helping them to refine what to look for. Kids often only begin to interpret and understand an assignment as they do the research. Because they don't have an easy set of categories to put the information in, they can get sidetracked intellectually.