You may never need this information, but if you do, it's good to have it. Take stock of your family's valuables. Inventory your safety deposit box or family safe and take photographs of the contents. Do the same with jewelry or any furniture, paintings, or other items of value. Property insurance policies can be very helpful here, because many companies ask you to list the valuables you want insured.
Some people keep a list of belongings in a safe, also for insurance purposes.
If you've done that, start with that list. There's no need to reinvent the wheel. Get detailed information on every policy you jointly or individually own. And remember, get the name and phone number of your insurance broker now. Learn how much it costs to run your household now.