Creating a prototype reveals the pain points related to creating the product, and it paints a clear picture of what will work and what won't AND how much the whole thing will cost.
Create a "prototype" of the business that you're interested in starting.
Want to start a marketing agency? Build websites? Get into home services? Dropshipping? Allocate a relatively small budget ($500-$1,000) into actually starting that business.
You'll make tons of mistakes, and you'll probably lose all the money you put into it, but you'll come out of the experience with a much more clear understanding of what it actually takes to succeed in the industry you're interested in.
Fight as hard as you can to make some revenue, and things will start to click. Then, rebrand and start fresh with all of the knowledge you just gained. Leave the angry customers behind and succeed now that you know what works.
When I first started a remote cleaning company, I thought about things like customer acquisition cost, how to build customer/cleaner funnels, and other KPI's. I DIDN'T think about what to do if a cleaner calls in sick at the last second, how to set up direct deposit for cleaners, and how to handle disagreements about the length of jobs.
Many of the headaches related to starting a business exist within the nitty gritty - things people on the internet don't talk about much because they aren't sexy. But these are the things you'll need to master in order to succeed in ANY business.
Plus, like anything, the more practice you get running your business, the better you'll be in every way imaginable. So just like any good inventor wouldn't mass produce their first idea, create a prototype of the business you're interested in before diving in head first. You'll be surprised how much you learn and how many mistakes you'll be able to avoid when you do the real thing!