Battle Garegga's self test routine is helpful in eliminating some basic problems. If video output is working correctly on the board, it will correctly identify a failure in main ROM, work RAM, video RAM, text RAM, and color RAM. If the game has at least progressed past this screen, you can be sure that those basic memories and their associated bus control logic are functioning. The bootup test will also attempt to inform you that the sound CPU is not functioning correctly, though the absence of such a message does not guarantee full functionality of the sound CPU system.
The self test will NOT test Sound RAM, nor very thoroughly test the communications between the main and sound CPU. Problems may arise that are symptomatic of issues with these areas, but the test may not directly report them.