I tried one of those hidden mirrors about 12 years ago. It kind of works, but it really is dependent on how much sweep your glasses have. If they really wrap around your face, the little thing does not work all that well.

I use a Take A Look mirror. Very adjustable and sells for about $19. Clips onto your eye glasses. Some of the racer wanna-bes look at it with scorn. I have just moved to the Czech Republic and am learning new roads and a new culture. I wear it here and it really is so easy and simple to have that I forgot why I stopped using one some years ago. Well, actually I know why, I lived in rural western Massachusetts where there just are not many cars. Back in the late 1970s, I used one of the Ed's Mirrors eyeglass mounted devices. Also a real quality item.