In practice
Customers stay, tell other people and would be genuinely upset if the product disappeared. There is no agreed measure, but useful signs include retention that levels out, unprompted word of mouth and loud complaints when the product breaks.
Having customers or revenue doesn't prove fit. Both can exist while every new sale is still hard work.
Why it matters for product builders
Almost every decision gets easier after fit. Scaling before it tends to multiply whatever isn't working.
What changes in AI-first products
Nothing about fit got easier. Building faster lets you test more cheaply, but it doesn't make the product more wanted.