The easiest way, if you have kept the invoices for your goods, is to add them up. In the (frequent) case where you have not kept the invoices or receipts, you can look up the approximate value of your belongings on the internet. If they are valuable objects, don't hesitate to have them appraised by an expert in order to know their precise value. In any case, it is better to slightly overestimate the value of your belongings when you take out insurance to be sure of being properly compensated in the event of a claim.