It is best if you have a computer that you can plug directly to your router. Do so by plugging an ethernet cable from the ethernet port on your computer to one of the LAN ports on the router.
Hover you mouse over the quick links in the top right corner or click the three horizontal lines in the top left corner.
This will take you to a page with a blue check my speed button, click that button.
This will take you to a screen with a round go button, click go.
The test will take around 30 seconds to complete. You will want to look for the numbers in the download and upload boxes once complete. Those boxes will show your current download and upload speeds.