Many of us do credit as many of the product and freebie developers as "humanly" possible. It is hard though because sometimes the item names, folder names and readme files conspire to make it very hard to find who produced certain (especially freebie) items. Another problem, that I have, is that I like doing complex scenes and I end up filling all of the product credit boxes.

Having said that, It really is far easier to ask the artist what a particular character or prop is, because then you don't have to guess which product credit matches which item in the image and the artist hasn't had to list everything that was used. I would think that most artists would respond very favourably to any information request.