Q. How do I know the articles I find on the internet are valid?
You are right that articles you find on the web can be difficult to verify. One thing to check is the web address. You should look for the .edu or .gov at the end of the URL. Check the author and/or publisher of the site, determine when the information was posted, and check for bias.
See this for more web evaluation tips: http://www.csulb.edu/library/tutorial/section3_reading2_internet.html
Another way to find valid/quality/scholarly articles on any topic is to use the library's databases.