The theory of product market fit has been diluted with too much "explanation" in essence the answer to your question is:
You know you have market fit because your product is being bought, shared, and highly demanded. This will in turn scale for you.. More demand means more production which means financing, hiring, adding to marketing, etc. don't scale if there's no sales. No sales means no product market fit.

That's the definition of market fit; you have created just what's right, no fluff, and marketed just in the right way that consumers get the value and want it.

Once you have a fit... Marketing is the best first option for scalability. Invest in marketing. Drive demand and sales and use that to fund increase in tools, labor or production.