The following are the primary ways GDRI tries to determine the developer (company) of a game:
An "actual mention" is when a company name is actually mentioned in or on a game. Many games or game packages will state directly that the game was developed/programmed by a particular company or companies.
Other actual mentions are not so forthcoming, however. They might say "Produced by Company" or "Designed by Company," or the company might only be mentioned in the copyright notice. In these cases, we will look closely for other evidence.
Another kind of actual mention is when the name of the developer is written on a wall, sign, or other object in a game. An example of this can be found in the unreleased Ninja Gaiden for the Sega Mega Drive. A sign flashing "Opus" backwards can be seen at the end of round 4-2. 
Mentions in credits are also considered actual mentions.
Thanks to a program developed especially for GDRI, we can perform code comparisons. This program can look at two different files and show what is shared between them. For our purposes, we can compare two ROMs for games we know to be by the same developer and see what code is shared between the two. From there, we can search for this shared data in other ROMs. If there is a significant amount of shared code in these other ROMs, there is a good possibility that these were programmed by the developer of the two games we originally compared.
This is not necessarily helpful if there is a lot of generic code used, or there are drastically different programming teams, but it can be a powerful tool, especially when backed up by other evidence.
The aforementioned program is not available for download yet as it is not in a distributable state.
Hidden data are data in a game that a player cannot access through normal use or through "tricks" (e.g., pressing a button combination to bring up a debug menu or special message). Other tools are needed.
Text strings are the most common type of hidden data. You can see them by opening up a ROM or disk image in Notepad. We prefer using Strings, a tool for finding UNICODE (or ASCII) strings in files. Example:
MS SONIC¥THE¥HEDGEHOG.2 Ver1.00 1992/09/05 SEGA /Aspect Co.,Ltd
Another type of hidden data are graphics. For example, if you view the tilesets for Donkey Kong, Congo Bongo, or Zaxxon in MAME, you will find the Ikegami Tsushinki logo.
GDRI will try to get information by making contact with former staffers. Look at our Interviews section to see many of the people we have successfully talked to so far.
If you are a former staffer with one of the development companies covered on GDRI and want to help, please contact us.
Company websites are great resources for information, especially if the company is open about the games it worked on in the past (thus eliminating a lot of legwork for us).
Failing that, an excellent resource is Developer Table, a Japanese site where you will find works lists for a multitude of game companies. If we are interested enough in a company listed, we might be inspired to do some of our own research.
Books, magazines, and other publications can also contain useful information. Check out the Further Reading section of a particular entry for a list of resources.
GDRI scours the Internet, ROMs, and other sources to find game credits. We might even play through an entire game just to get them. When we have gathered enough credits from various games, we can compare them, see what is shared between them, and perhaps come to some sort of conclusion.
As we mentioned before, credits can be found in ROMs. These are usually the same credits that can be seen by finishing the game. This is not to be confused with text strings as described under "hidden data," which are typically not meant to be seen by the general public.