The only way to know what brand is a media is to read its ID.
Umm, I *REALLY* wouldn't suggest that to people anymore... I happen to know of several manufacturers who are very soon going to be releasing DVD media with several different codes. We already have seen TDK and Maxell codes on discs not made by TDK or Maxell, but soon we will be seeing discs with Taiyo Yuden's code on them, and from more then just one or two manufacturers... So I would say knowing your discs, and knowing your vendor is the best way to know who made them. Recognizing serial numbers, and buying trustworthy brand names is also a good idea.
As for Samsung, they used to use Ritek quite often for many of their products, however they joined with several other large brand names and ditched Ritek when their DVDR quality started fluctuating too much.