I just recently started to realize that the question is much deeper than it seems at the first glance.
Imagine you are in the middle of 1990s, looking at the Internet
you may think it's already done/finished and there is nothing big you can invent.
You have perfect Netscape browser and very convenient Yahoo along with cool AltaVista.
If you were being told about yet another brand new search engine (Google),
you may think - why those guys are wasting their time, there is no chance to beat Yahoo.
Yet, Google became monopolist amongst search engines.
Then you watch how dot-com bubble burst in 2000.
Then you may think, the Internet is done for sure.
There will be no second company which grow as fast as Google did.
Then, you will observe rise of Facebook.
In the beginning you won't believe it could grow THAT big,
after all it looks like match.com for Harvard students.
Then you will discover somebody wrote Bitcoin.
You won't even recognize it's a currency
because it's just tokens in p2p network with no connection to real world.
Then after reading stories about Silk Road,
you finally started understanding that you can use as a currency literally anything
as long as it hard to reproduce and in limited supply.
But watching Bitcoin crash in 2011, you may think that Bitcoin reached its limits.
In beginning of 2015, you would think crypto-currencies are just a bubble.
Nobody in their right mind would spend their time and effort to create yet another crypto-currency.
Yet, Ethereum started in 2015 and quickly become the second largest crypto-currency.
I tend to always think it's too late and market is already done.
So returning back to my question: How do you know you are still in early market?
As an answer, it would be awesome to have some references to insights from some founders or VCs.
One strategy is to make a habit of staking odd, unusual or long-sighted ideas. You would lose 9 out of 10 stakes, but hopefully the tenth would cover the losses.
I found out about bitcoin around 2010, back when they were still almost CPU-mineable and cost a dime a dozen, and mined a few "just in case they would become galactic credits or something". It sounded like a bizarre idea, but i couldn't think of any solid reason why it can't just became money one day. Later, i got a bunch of Ether because i had a habit of collecting altcoins, and it seemed like something unusual compared to the meaningless forks abound back then.
Sometimes you need to understand the tech. I groked the idea of cryptocurrencies quite early, because it's just math and code. But i wasn't aware of Facebook's existence until around 2014, because social networks were some bizarre, unsearchable chat-forum mutants with zero information content or conceivable uses. Even though i am as old as Zuckerberg and had all the knowledge to make one, an idea of social network would never have occurred to me.
So, there is some degree of personal alignment and aptitude - if you are not in a certain field, it's hard to get what can have traction and what can not, but if you are familiar with a field, you can detect unusual ideas with potential more often than not.
TL;DR: Kinda sorta something like that, no real answer, didn't meaningfully succeed at it either.