You produce evidence, regardless of what form it's in.
For example, if you plan on selling stock, produce your stock broker's statement.
If you literally have it in cash, you might be better off putting it in the bank to prove it's somewhere.
If it's in the form of precious metals--say gold--produce evidence (such as the depository statement) showing the quantity of gold you own.
Those are just a few examples.
Hope that helps.