There is no magical answer; however, here are some tips to help to identify legitimate opportunities while avoiding schemes.
Ask yourself, "It is too good to be true."
Most things presented as requiring little to no investment, no dedicated time on your part and promises that you can make a tremendous amount of money while doing nothing are usually not legitimate ways to make money. If it sounds too good to be true, it probably is.
Beware of turnkey business offers.
Online businesses that are already set up and ready to make you a lot of dough are usually either scams or schemes. While legitimate companies sometimes offer a free website to promote their products through, most turnkey businesses are not legitimate.
I do however believe that it is possible to buy websites for very little that have great money making potential. In fact, I have bought two that were not making money. I tweaked them for the search engines and added keyword focused content and they were soon profitable. Where you have to be careful is knowing whether you can do those things.
Be cautious about buying money making websites.
If the website is making money, why would the owner be selling it? Some website owners sell their sites because they are changing their focus or they don't have the time to handle the business. Don't pay asking price without documented evidence of traffic and profit. You can make money two ways by buying a website: (1) You can buy one that is already making money and pay what it is worth to you to have the existing income stream. Expect to pay a lot for that. (2) You can buy one that is not making money, but is one that you believe has potential because you know something the present owner doesn't. Expect to get those cheap. Owners will often take your first offer. Make sure you have done your homework.
See what others think about the opportunity.
Never trust solely in testimonials of the website owner that is presenting the offer. Testimonials can be faked. Instead, visit money making forums and ask people if they have any experience with the company or just run the name of the company through your browser.