How can I check toner levels.

For the most part, you can't. Toner isn't like ink. Ink is a liquid, toner is a powder that is heated up to a liquid via the fuser when you go to print something so it's not as easy to tell what the toner levels are. In fact, a common "trick" with toner carts when they get low is to tilt them from one side to the other in a kind of see-saw fashion to get a couple dozen/hundred additional pages.

Which is the good thing about laser printers. They are ridiculously more efficient than inkjets. Inkjet carts are measured in dozens of pages, lasers in hundreds or even thousands. So it's not nearly as critical an issue. By the time the printer starts telling you the toner is low, you'll probably still have a few hundred pages to wait for a good deal on a replacement toner cart. And also remember that the cart that came with the printer is only about half-full, so figure that any replacement will last roughly twice as long.