I havent found such a register.
You could try to write to a flash memory location which does not exist for the 1 and 1.5 MB version and check the result. If it works without an error, it's the 2 MB chip. Otherwise start the next test to differentiate between 1 and 1.5 MB versions.
You could try to write to a flash memory location which does not exist for the 1 and 1.5 MB version and check the result. If it works without an error, it's the 2 MB chip. Otherwise start the next test to differentiate between 1 and 1.5 MB versions.