One FAQ is "how do I find the address at the end of code?"; or, similarly, "how do I find the end of my variables?". The usual answer is to add a small segment, tell the linker to locate it as the last segment, and then take the address of that object.
So I was a bit surprised to find this page in the LX51 manual:
http://www.keil.com/support/man/docs/lx51/lx51_ls_classinfo.htm
Seems like LX51, at least, does automatically create symbols for this purpose.
I haven't tried the feature myself.