How to decide when your blades need to be sharpened?
Also consider buying a new set. That way you'll have sharp blades to use while the others are in the shop. It's always nice to have a spare pair.
About the cutting edge, I see what you guys are saying about the light or a different shade on part of the blade. I used a flashlight and this worked pretty good. How much of the blade can be compromised where you see a bit different edge/cut on it where you'd still be ok with it?
I'm using an ION and want to minimize extra work for the machine if possible. If it were a hand auger like I had used in the past, I was a little more tolerant of a dullish blade.
Look at the exact cutting edge. You'll see some wear. Light will shine off the worn,"rounded" edge just a little bit.
It should be pretty obvious and you shouldn't really need the magnifier - especially if you've got another sharp (extra) pair of blades to compare them to....But if I remember right,you ain't got the eyeballs of a spring chicken neither,so magnification can't hurt.
Now, my new Ion electric ice auger shaver blades are a different animal for me to sharpen. Will see how that goes.