Took a closer look at the volume behavior in SPY and if we were to use the previous Nov run-up as an analogue, we would like to see some evening in volume between up and down days before getting aggressively short.
The green zone represents area where the up days are marked by much stronger volume than down days.
The yellow zone represents areas where the volume is starting to increase more in favor of down days.
The orange zone for periods where down days dominate on volume.
Basically the green zone still presents possibilities for shorting but high risk of buyers moving in so one would need to be nimble and take gains very quickly. i.e. within a day or two.
The signal I would looking for would be a red volume spike on the daily chart that represents more aggressive selling coming in. So far though on this up cycle it hasn't happened yet although it could come out of nowhere and perhaps bypass this yellow zone and go straight into selling.