There is no way of knowing in advance, and if you have 100% certainty by the time you take action it will probably have changed already.
The strategy for that is to try lots of experiments, and make sure you know early enough if they are going to work. (Months rather than years) When you see that it works, pursue those experiments that look promising.
On the flip side, you'll have to expect that quite a lot of experiments don't pan out. We are conditioned in school to avoid risk so this experimental mindset is very difficult for many!
When you do experiments, you need to be sure you can accept the worst case scenario. If you can't accept the loss, you are speculating or gambling, that's not the same.
This is where the problem exists with experiments like a college education of 4 years for x amount of money.
Where before the likelihood that such a bet would result in a decent job was pretty good, now that likelihood has gone down.
Not saying that university and higher degrees aren't useful, rather that they stopped having the almost universally positive result for many people.
The worst case scenario is coming true for many: spent 4 years of non-productivity, paid several 10's of thousands of dollar/ €/GBP and no chance of a job that pays enough to pay that back.
Totally agree