Think About What You Want to Know
Before you start reading anything, ask yourself why you're reading it. Are you reading with a purpose, or just for pleasure? What do you want to know after you've read it?
Once you know your purpose, you can examine the resource to see whether it's going to help you.
For example, with a book, an easy way of doing this is to look at the introduction and the chapter headings. The introduction should let you know who the book is intended for, and what it covers. Chapter headings will give you an overall view of the structure of the subject.
Ask yourself whether the resource meets your needs, and try to work out if it will give you the right amount of knowledge. If you think that the resource isn't ideal, don't waste time reading it.
Remember that this also applies to content that you subscribe to, such as journals or magazines, and web-based RSS and social media news feeds – don't be afraid to prune these resources if you are not getting value from some publishers.
Know How Deeply to Study the Material
Where you only need the shallowest knowledge of a subject, you can skim material. Here you read only chapter headings, introductions, and summaries.
https://www.mindtools.com/rdstratg.html?