In practice, that means I'd visit your residence or residences for some days and view the collection in context. I'd take pictures with a digital high-resolution camera and create an inventory of your collection or collections. After consulting with you in detail and working on it, we produce a booklet of the collection, with digital images, description of objects and their approximate value. The description includes size, medium of the art, provenance, condition, etc. The value is not an official appraisal.
Most collectors are funny about having the insurance company appraise their property.
Also, a proper appraisal is done by a specialist in the particular area of collecting. But we base our rounded figures on original receipts, documents of authenticity, the index of rise in prices for the particular genre of art, all of which is a good way to recollate lots of dispersed documents. You'd be surprised how many people collected their art or objects years ago and haven't updated their inventory or dug up the relevant documents. Some collections, which might be very eclectic and include diverse genres, are spread between several houses, so it's a chance to organize it all. Often, people have inherited their collection and they don't know the scope of it.