👨‍👩‍👧‍👧 Team

For me, team is the most important thing to look at when assessing an early-stage investment opportunity. The idea may not be fully formed, they may not have product-market fit, but if the team or good, there's a chance they'll succeed.
First, I look at the background of the founders and see if there is a logical narrative as to why they would start a business like this. Expertise, in this case, helps - especially if the business is very technical. Good education and experience at good companies are useful signals that the person might be driven and/or capable, but you can also fall into the fallacy of believing that just because someone worked at X company, they are going to be a good founder. This is dangerous. Also beware of the cognitive trap of overestimating people because you self-identify as part of the same "tribe" (whether that's about interests, race, gender, professional background, class or anything else).
To avoid this trap, personally, I'm most interested in looking up the person on social media and digging into what they have done and written / spoken about. It gives you a clear picture of who they are and how well they market themselves and their business. It also lets you search for the secret sauce: passion.
If you speak to them, you can get a real feel for their passion, which is crucial. But you may also be able to see traces of this passion (or lack thereof) in their behaviour online. This lets you decide pretty quickly whether or not to give someone a chance
It’s also crucial to understand team dynamics between founders. Founder fighting is one of the main reasons companies fail.

💹 Market

People talk a lot about "product-market fit". It's a good heuristic, but you have to be careful about this term. I try to assess the two separately, and I start with market. Why?
"If they have product-market fit, you're already too late". Unearthing great opportunities (and capturing value) is about sensing the signals before the company has really taken off. Startups usually meander their way to product-market fit via several failed iterations (this is why team is so crucial).
What is important when it comes to market is momentum - if you have a good team and they are operating in a space that is growing quickly, it will be easier for them to find a way to make it work. If their target market is shrinking, it will be much more difficult. Size of the market now is not as important as how big the market might become in future.

❤️ Product

Do people love it? For me, it's more about whether the people who do use it are superfans than how many of them there are.