TOURISTS drove six hours to Mankato, Minn., in search of underground caves and hot springs mentioned on a Web site. When they arrived, there were no such attractions.

People searching for a discussion of Amnesty International's views on Tunisia learned about human rights in that North African country -- but from supporters of the Tunisian authorities, not from the human rights group. The government supporters brought surfers to a site with a soothing Web address: www.amnesty-tunisia.org.

And bibliophiles who trust the grande dame of on-line retailers, Amazon.com, for suggestions under the headings of ''Destined for Greatness'' and ''What We're Reading'' were dismayed to learn that some publishers had paid for special treatment for their books -- meaning a more accurate heading would have been ''What We're Paid to Say We're Reading.'' (After the disclosure, Amazon added a note on its home page to make a subtle acknowledgement of the practice.)

On the World Wide Web, straight facts can be hard to find. After plowing through dense and recalcitrant search engines that offer more sites than you can point a mouse at, after enduring delays, lost links and dead ends and arriving at a site that looks just right, Web surfers must deal with uncertainty: Is the information true, unbiased and free of hidden sales pitches?

Even though it is easy to fall prey to parodies, politics, payola and ignorance on line, solid, watertight information can indeed be found on the Web. But experts on Internet research point out that the Web is largely unregulated and unchecked, and so they agree that it is wise to be skeptical: Consider the source. Reconsider the source. Is the information up to date and professional and traceable? Can it be verified, or the source checked, off line? And just who was that source again?

Don Ray, a freelance investigative reporter in Burbank, Calif., and the author of ''Checking Out Lawyers'' (MIE Publishing, 1997), has what he calls a J.D.L.R. test to apply to Web research. ''There should be a switch in every Internet user that toggles when something Just Doesn't Look Right,'' he said, ''to make them re-evaluate the credibility of the source.'' If a Web page has grammatical errors, sloppy spelling or a goofy design, that makes him distrust the content.

And people who are getting ready to spend money on the basis of Web information should, of course, approach their decisions with at least as much skepticism as they would use about a purchase off line.

Whoppers have found a home on the Web since the very beginning. Yet for many people, computers have generally been treated as authority figures, able to calculate compound interest in a single bound. A machine that has been perfected by institutions of higher learning and is relied on by the Government isn't likely to lie, is it?

''We've inherited this notion that if it pops up on a screen and looks good, we tend to think of it as fairly credible,'' said Paul Gilster, author of ''Digital Literacy'' (Wiley Computer Publishing, 1997.)

Although the Web has come to resemble a monstrous library system where everyone has a printing press and all information is seemingly created equal, even the newest surfers come to it with useful information-sorting skills from the off-line world. They can differentiate among information from a trusted newspaper, a bulk mailing from a charity, a sales pitch from a stockbroker and a letter from a friend. They can distinguish commercial broadcasts from public television programs. They can skim over the pages in Reader's Digest with ''Advertisement'' printed at the top.

But on the Web, the clues for credibility are different, and so are the tools needed to assess the information. How can someone know if a favorite portal site is making a nanobuck in sales commission every time the person buys something at the florist featured on the page? Comments from people who are either touting or trashing a stock on the Web for their own financial gain have been investigated repeatedly by the Federal Government. And is that medical information on that site underwritten by a drug company or by someone on drugs?