Most of the world's investable assets (real estate, oil wells, corner ice cream parlor, a Taco Bell franchise) have not had a liquidity event but are valued nonetheless.
However, it's still far from a done deal as each transaction has its own minuscule details, and in commercial property, e.g., one party might value such things as location, quantity/quality of parking spots, proximity to traffic flows, quality of neighbors, recently incurred maintenance, etc. differently from the other.
With that said, a 409a valuation firm (or potential acquirer) is likely to look at the peers, current and historic ones, to at least start at some ballpark figure.