There probably isn’t an Art topic that you can’t Google or can’t YouTube and not find either a free tutorial or an article about. I have done my fair share of searching, for instance I have looked for:

How to draw Manga characters
Digital Rendering
Abstract Art
Cartooning
Animation
The results of each search were numerous, doujin pages and pages of them. There is no precise way to determine which ones were good, or which ones were what I was looking for without opening and watching them for a while.

I started studying art in 1996 and have not stopped since, there are always new things to learn. I have learned from anyone that has come close to me and while I know that learning from peers or people around your own level of knowledge and expertise is important, I also know that peer learning works best when you already know the material. In other words, if you are a beginner, try to pay attention to people who actually know what they are talking about, and not so much to people who are also trying to figure things out.

Anyone can post a tutorial and if you pick the amateur tutorials, your progress will be minimal to none. The Professional Tutorials are usually not free, when they are they show you briefly how to do something and then direct you to their website so you can purchase a complete version of the tutorial you are interested in. For a while I wasn’t sold on that idea, but then I started to put myself in their shoes and realized that I also would be doing that to make a living, and if what they are trying to sell is what I need, then there is nothing wrong about it. I appreciate the Free Tutorials that show you enough for you to determine if that is what you need or not, so then you can decide to purchase it. Free preview tutorials are not the only good ones out there, I have bought tutorials without preview too, based solely in the description. Most of the time I get it right, but I guess I have an eye for them.

In conclusion:

Don’t expect a free tutorial to contain every bit of information you need about a topic.
Don’t expect a free tutorial to always be professional.
Expect the professional free tutorials to be a link to a paid more comprehensive one.
There are some free tutorials that are good but often leave so many gaps in their message that you need to go and find another tutorial to compensate.