1) what is the current size of this market?
2) what is the size of the total addressable market?
3) who are the main competitors in this market?
4) what are the product/service gaps in this market?
5) what are the known user metrics in this market?
I usually start by searching Google. This typically leads me to SEO optimized blog posts, where companies have cherry-picked specific market data to justify their own products. The data is usually from industry research reports (e.g Gartner, Forester) that cost thousands of dollars to access the full versions. In early stages of ideation, it doesn't always make sense to spend so much.
How do you perform market research for your ideas?
G2 is another avenue, albeit at a less scientific level. It gives you a leader/loser chart similar to the magic quadrant.
For anything more detailed, you pretty much have to pay up to the usual suspects of market research firms. Maybe you could form a syndicate of other interested parties to contribute to the full cost?
So I figure the shortcut is to look for competition first. You want to see something in a large market, which makes you scratch your head why such a shitty app has so many users. Or if you think customer service agencies are doing such a horrible job that a robot can be more polite.
The next thing is ask yourself how you can make something 10x better than it is. This is where tech really comes in - if they have a long development roadmap and are slow, see if you can use tech to outpace them. If they're being held back by a middle man partner, see if you can use tech to automate the middle man (like taxi companies). Or if you're lucky, it's an app with hundreds of thousands of monthly users that crashes all the time.