Well it depends on the asset. If it's regular corporate bonds that are being actively traded on an open market (liquid) then just use a recent market price. If there's no market then valuation becomes highly subjective. If the borrower has some kind of credit rating from a trusted rating agency then that can be used as proxy for default risk. Or if the borrower is publicly traded and releases audited financial statements then you can estimate based on those. But if you don't have any of that then pick a number and try to get the auditors to swallow it.