MemTest 64/128 v1.10 by Robert Olessak (released on 02/20/2020) is a memory testing program for the Commodore 64 and 128 that tests not only main memory and vdc video memory (C128), but can also test external memory expansions including the Commodore REU, GeoRAM, SuperRAM and  so on...  The latest version of the program can be obtained from http://istennyila.hu/stuff/memtest.zip

Inluded in the archive is full documentation and source code.  Here is a link about MemTest over at the Lemon64 forum.