You would write to and read from every memory block. For example, in an 8-bit RAM chip you could write 0x55 to address block 0 and then read it from address block 0 to verify that the data written matches the data read. Next you would write 0xAA to the same address to toggle all the bits and run through the same read/write cycle. Continue for all RAM memory blocks of the data read always matches your RAM is good.