There's a question of how to judge the value, but that's not much of a bigger issue than for regular capital gains.
Regular capital gains is literally just "what you sold for minus what you paid for it". For any major capital asset, both those numbers should be well documented. Judging the value for realized capital gains is literally subtraction. There is no subjective judgment involved, there is no guessing, there is no issue.
It's a bigger problem for such enormous wealth to sit in unutilised or underutilised assets.
Nearly all of the enormous wealth of billionaires is invested in corporations. What asset is being underutilized? No cash is being locked in a vault, no physical commodity is being hoarded, and if they were to sell or give away all the stocks, they would just be owned by other people, the businesses they represent ownership of would not inherently be effected by changing hands.
