how do you value an asset?

Answer 1

Assets are if two types, Capital Assets and Assets that are fungiable like Gold.
Capital Assets are valued using the CAPM or Capital Asset Pricing Model. It can also valued using the Discounted Cash flow model whose discount rate can be found out using the above CAPM or can use ROI, cost of capital or opportunity cost rate meaning subjective rates.
Fungiable Assets are priced based on the demand and supply of the asset.

Answer 2

It depends. [That is the standard answer all CPAs are advised to give until they have the details.]
Are you determining a valuation for tax purposes, for financial statements, or for buying/selling an asset?