You never "know" anything. If you buy something, it could be the top and you lose. If you short something, it could be the bottom and you lose. But if you stick your toe in the water and it works, try a foot. That's what transitioning means. I suppose you want something more technical (and useful) than that, eh? Well, it's not a simple answer.
You can't see a crash coming (1929, 1987) though some here have claimed that they can. However, a bear market is different and you have to be a bit oblivious not to recognize that something is happening when the market drops 50+ pct over 18 months (banks failing left and right, GM facing bankruptcy, investment banks collapsing, etc.). You'll see many market metrics deteriorating - economic indicators turn down, earnings announcement disappointments increase, analyst downgrades and earnings revisions increase, market leaders cease to lead, the VIX increases... all beginning long before the crisis becomes acute. When you decide to alter direction, you may very well sell at the bottom. You never "know".
Since I'm older and I have accumulated my nest egg, it's more important to me to keep it than to make it, though I'm not averse to making it

. The market going up without me bothers me a lot less than it going down with me and if I'm wrong, since markets don't "melt up", I can always get back in. So OK, I might miss a bit of the upside and pay some taxes on closed positions but the 50+ pct hit isn't on my menu, well, not since 1987.
I was clever enough to get out of the way in 2000 but not experienced enough or brave enough to go short big time. In the fall of 2007, I saw a number of troubling metrics. I simply determined how much of the previous 6 years of gains I was willing to give up and I ran a mental trailing stop loss. That trigger hit and I was out of most of my long positions by 12/31/07. From there, I ran a long/short correlation portfolio for the next two years, biased more short than long. It paid off well.
Maybe I got lucky twice. It doesn't really matter to me whether it was luck (a blind squirrel finding an acorn) or skill. I was willing to be wrong but luckily, I wasn't. As an example of my frame of mind, I started a chain about two weeks ago, asking for index hedging suggestions for some appreciated portfolio positions that are reasonably correlated (a variable annuity with a near double and some other managed money). This time, rather than blow everything out with a mental trailing stop loss, I'm trying to figure out a viable way to keep some upside yet have a floor (a collar) so that even though it's unlikely, a crash is covered as well. And there it is again, willing to be wrong small but not willing to bite the dust big time.
Hope this helps. If not, "Look Out Below !!!"