I'm new to 70D, coming from a Fuji mirrorless system. I'm used to being able to see my exposure change in my viewfinder as I change settings on my camera. I cannot figure out how to make the 70D do this. Can anyone help and offer guidance?
See What is it called when aperture and exposure are previewed on the LCD screen before the shot is taken? — Canon calls this feature "Exposure Simulation", or
EXP. Sim. Of course, you can't enable it in the viewfinder, since the viewfinder is showing you actual light coming in through the lens directly — it's not an EVF like in you Fujifilm. But, you can enable it in "Live View" mode for the rear LCD. (See page 230 of your manual.)
Shutter speed and ISO can't (for now?) be simulated when looking through an optical viewfinder. BUT you can see aperture variation using the DoF preview button on your lens.