A screen reader is the only gold standard test in knowing whether a webpage is accessible or not.
Using a screen reader is a skill, but it is worth at least experimenting with it to have a basic awareness of what those who depend on them need to do to access the internet.
These resources can give you a basic overview and help you get started experimenting.
Printable quick reference guides
Before downloading and activating NVDA on your computer, it is wise to print out some keyboard shortcut guides and at the very least know how to close the program.
LEANWI NVDA Keyboard Commands cheat sheet
Deque University Quick Reference Guide: NVDA for Windows Keyboard Commands
Deque University NVDA Keyboard Shortcuts webpage
Downloading NVDA
First, be sure you have a keyboard shortcut cheatsheet printed.
About NVDA (Non-Visual Desktop Access)
Getting Started with NVDA - Videos
Working With NVDA - Videos
Practicing with NVDA Screen Reader
Here are some links to pages known to work well with a screen reader so you can practice navigating successfully on accessible webpages (this does not imply promotion of the products or services on a given page, only that the page itself demonstrates accessibility).
Practice navigating accessible websites:
Practice navigating accessible calendars:
Practice using accessible forms: