Too many teachers dont' know the secret formula for "rich text" and we as librarians can help out here.
Lexile measure is only a small piece (although vital) to the equation. The other components are: How valuable is this? Will the kids get excited? Does this passage embrace rigor where the students will have to read closely to understand and grasp difficult concepts? AND -- What am I going to ask the students to do with this passage?
Our knowledge of database searching will prove valuable in finding great articles. Some lexiles are front and center in a search, while others require the use of advanced search features to find Lexiles.
When evaluating the "richness" of a piece it helps to have a rubric. Here is one I've developed, and two other links to NYC's rubrics. If you wrap your head around this, you'll be in good shape.
You can find the 'true' Lexile measure of a passage directly at the company who owns the trademark, Metametrics, Lexile.com Open and account, and upload your .txt document (less than 500 words) and get the correct count.
See my previous blog posts for additional information on this topic: