I learned about the 5 finger rule on my first day at work. It's pretty basic and easy to remember. The idea is you hold your hand up and read a page from the book you have chosen. You raise a finger for each word you don't understand and, as shown in the image below, if you have 2 or 3 fingers up, then this book is just right for you!
This strategy is great for Primary School kids since a lot of the times they will choose a book based on the cover (whoever says don't judge a book by its cover hasn't worked with kids!) and this also works when you're learning a new language and you want to read a book in the new language you're learning.