That's a good question. I think the best way to answer that is through experience and chart patterns. I do consider the overall market sentiment and I like using a mixture of technical and fundamental analysis such as looking at support/resistance and matching that with expected events like fomc meetings or at the moment trade talks/wars etc.
More importantly, I will look at the individual stock's chart for the past 6-month, 1yr, 2yr, and even 5yr (weekly). In these charts I look for repeating patterns whether there are 3 day rallies, post earning reversals, or simple cup base breakouts and see how often that occurs throughout it's chart. For my positions, once they reach about 200% in gains, I'll take a closer look for an exit quick should things reverse. I don't know why, but through all the trading I've done, that 200% gain seems to be a magic number for everyone else too, so I keep that in mind. Otherwise, I do look at previous patterns to see how far a certain stock will run before stopping.
Here are a few examples of what I'm talking about. I played a call spread on INTU heading into its May earning report. Fundamentally, I expect everything related to taxes to be all online soon, and INTU plays that entire business online. Looking at chart patterns, I saw that INTU has dropped post earnings in Aug and Nov 2017. The initial drop didn't last long and INTU rallied back to a 7% gain from the bottom right after reporting its earnings. Playing the call spread, I can leg out of the short side near the bottom and then let the long side run until INTU reaches about 7% from its bottom. In May 2018, it did just that and I took the short out near the bottom while playing the long side up to about $200. This is the reason my favorite quote is "history repeats itself." It's because we're all human, and sooner or later we end up repeating history. The same can be applied to stocks, and that's why you get these chart patterns that repeat forever. Sure, it may not be continuous, but you can bet that a certain pattern 5 years ago will reappear. The key here is to figure out which patterns belong to which stocks.
I do look for an exit if there are wild swings in the positions I hold. If the rate of increase on a certain stock slows dramatically and volume decreases after a huge run, I will look to exit that position if the gains are high enough. I'd say if the gains on my options are about 100% are higher and I can see that volume has decreased and the rate of climb has decreased, then I will close out the position if it was a short term position (1-2 weeks). On something where I'm holding a longer dated option like what I did with HES earlier this year and I still like the chart's pattern, then I'll close out some of the calls (about 30-50%) and let the rest ride through the storm.