Prepare a list that describes each item as precisely as possible: type, make, serial number, distinctive features, likely location and last confirmed holder. Photographs from the home, invoices, insurance records, account statements, messages and first-hand witnesses can help establish origin and later possession.
Counter-arguments belong in the same timeline. If a gift is alleged, record when and how it is said to have occurred. If the item was kept for someone else or jointly used, distinguish delivery, access and any request for return. Keep originals intact and preserve digital photographs and messages with their date and source.