Marital assets are valued as of the date of separation. For high-value items like homes, real property, works of art, jewelry, or a business, it is best to get an appraisal of the property as of the date of separation. Of course, you and your spouse can agree to use a different method of valuation – the tax value, for example. As long as you both agree, an alternative method of valuation is fine.
Ordinary household items are valued at fair market value; that is, the value the item could fetch on the open market, if it were sold on Craigslist or E-bay, for example. Smaller items are valued at yard-sale value.