Earlier quoted context omitted.
How do you know you have over 100m in assets? One never really knows the worth of something until it's sold. (i.e. try selling a used car. there's what you think it's worth and what you get...) And once the asset is sold, that's a taxable event.
Easy. Share price x number of shares.
1. Assumes the asset in question is publicly traded.
2. Assumes the publicly traded asset has a non trivial amount of trade volume 3. Assumes asset price is relatively stable, moving in a narrow band along a clear trend-line
4. Assumes you have defined the price from the stock information (last trade before close. Daily average, etc)
5. Assumes holder's position is small enough not to affect stock price were they to sell.
And stocks are the easiest to do this with!
Look at the Trump vs NY court case for the value of his house in FL. Unlike the valuation imposed by government fiat, the valuation was agreed to freely by the parties. The courts found it excessive (and it might be) and proposed a valuation so ridiculously low it alone gives Trump grounds to appeal that the judge is either incompetent on the matter or has a personal bias and should anyway have recused himself.