Timing a market crash is impossible for average Joe....but it all really boils down to what you hold, your timeline and your risk tolerance. Pull it all to cash or bonds now....guaranteed to make nothing. Market seems hot for sure...but does it have another month to run? A year? 3 years? Stuff that seems overvalued today might seem cheap in a month or a year (thinking of all the real estate I should have bought over the years).
Are you holding mutual funds, precious metals, ETF's, individual stocks, all of the above? US based, or global? Given that....would you be thinking of a global downturn, or more specifically US? Certain things are still "down" significantly from all time highs....many CDN energy companies, oil companies, oil itself, yada, yada. So it really depends where you think that's all going. Some industries are just on the upswing....as we prep for legal weed in Canada, are some of the still small-ish companies prepping for that the next alcohol and tobacco companies?
The diversified stuff will likely feel a market downturn, but you could argue that certain individual companies may not be affected much by a down turn. The precious metals (such as gold) tend to rise during times of uncertainty. It's always a tough call....my best guess is that if you have a 10 year plus timeline for the money, and diversified holdings you are probably good either way.
Selling high and buying low is everyone's goal...but timing it is largely luck without being a full time stock guru. Even then....does your stock advisor end up with better overall returns than a small selection of good mutual funds? Many don't. If you are playing your own small selection of individual stocks....you are likely a risk taker and your guess is probably as good as anyone elses. I think my stock picks are great, but they don't always turn out great....or may take some time to do so.