To enter Service Mode:
The amount of memory is at location 2A8, so type this to see it:
12a8[return] 1[return]The first two words of the data are the memory size in bytes, as shown below. Note that unlike intel chips, the 68K is big-endian; the high byte comes first.
More than 2MB is only possible with revision 3A ROMs, and CPU and memory board modifications.
Source: James MacPhail / LisaList 2006.06.26[LisaFAQ] > [Hardware] > [Memory] (Comment on this answer)