As I understand it, you have photographed the products against a white background. I guess you're extracting the product from that background and pasting onto a true RGB(255,255,255) pure white background in photoshop. I think what you're saying is that you have some areas from the original background which isn't pure white, and those don't blend in with the pure white background?

If so, and the original has a color tint, and you want it to be pure white, then before extracting the product from the original background, use a curves adjustment layer, and use the white eyedropper and select part of the background and that should correct the color cast, and will also bring that part of the background close to pure white so it matches the background you're pasting on.

Answering your question more directly I think, once you have the image pasted on the pure white background, if you want to check it, I would use a levels adjustment layer. Move the midtone (middle) slider almost all the way to the right. This will increase contrast in the lightest tones and show you places where you don't have matching whites. You could also use a threshhold adjustment layer. Find spots, paint them over with a white brush or mask them off, then throw away the levels/threshhold layer when you're done.

I hope I've understood the question. If not, clarify :)