On the original ZX Spectrum, you could measure the write bandwidth visually, because on startup it would write the value 2 into each byte in memory (which included the graphics RAM). It would then re-read and decrease the value of each byte twice, to check for any faulty memory.

You could see these patterns on-screen as the reads and writes took place (I think it took about a couple of seconds to do this to 48k of RAM)