In identifying a painting once in Charles I's collection a number of things have to be considered. Although an important aspect of the identification process, the quality of an art work shouldn't be the deciding factor in assessing potential Charles I provenance. Some works occupying important positions in Charles I's palaces are today considered second-rate and relegated to the store rooms of museums, galleries and houses.
It might be supposed that all paintings owned by Charles I appear in at least one of the two major inventories; however, there are a good number that were given away via gift or exchange and that we know by other means formed part of the collection. Despite this caveat, a match in the inventories is one of the strongest supporting factors for identification with the collection. Matches can be more certainly made with van der Doort's inventory due to its lengthy descriptions (including whether the light is 'right' or 'wrong', meaning actually left or right respectively) and fairly accurate measurements. Note, though, that paintings have not always remained the same size throughout their history and often received additions or subtractions to suit the needs of later owners.
Firm matches with records in the Sale Inventory are inevitably more difficult due to the comparatively scant amount of detail. Therefore, matches with the Sale are usually determined by other factors such as a firm match in van der Doort appearing in a similar location in the Sale and bearing a relatable description, or through the matched painting bearing a 'CR' or the rarer 'CP' brand on its reverse.
Despite evidence of a number of fake brands appearing on the backs of paintings, the discovery of either brand – the 'CR' standing for Charles when king and the 'CP' when Prince of Wales – is still the simplest indication of Charles I provenance. Occasionally, the brand is accompanied by a contemporary note of van der Doort's containing information about the picture and some of the works that came from Mantua still bear notes reading 'Mantua 1628'.
There are, however, paintings known to have been part of Charles's collection that bear no brand. These are almost always canvases, which have been relined and therefore lost their marks of ownership – though some brands have been discovered via infrared reflectography. Whereas most brands that survive today exist on the backs of panel paintings, some cradled panels have also lost their brands. Furthermore, with such an extensive collection, it is perfectly plausible Charles I didn't have the opportunity to brand each of the paintings in his ownership. Most of the Mantua panels bear a brand, and it can be assumed they were branded on arrival. However, some of the older, inherited panels, known in the inventories as 'Whitehall pieces', are curiously brand-less.