There is no doubt that the broad US market is fairly richly valued.

But it is spectacularly difficult to estimate by how much--a little? a lot? an insane amount? what is normal?
One probably can't ignore the fact that valuations have been drifting upwards for a century and assume that the VERY long run average is the only thing that matters.
More likely you can only define "normal" valuation levels based on comparisons to the average since X years ago, and see what different values of X imply.

Lately I tend to use around 25 years to take a stab at the "new normal".
The market has been at its modern high valuation levels for much of that time--three bubbles if you listen to most folks--so the amount of overvaluation it implies is unlikely to be too large.

Jim