You can easily get within 10% of the "real" value on most assets. And, in particular, assets like stock have a built in ticker to tell you their exact current value.
This sort of evaluation happens all the time privately. For example, car insurance companies have gotten extremely good at evaluating the value of a car to determine when to simply total it.
The only thing that really makes it tricky is hidden assets or assets with no market value.
The likes of the richest people, who I think most of the "tax wealth" people are thinking of, have the majority of their wealth in equity. It's easy to tax the majority of their wealth.
This does not need to be a perfect system to be very effective at generating revenue and redistributing wealth.