It appears that there is no surefire way to tell, however various approaches can get you some sort of answer. Apparently you pretty much have to try the different ones until you find one that tells you ECC is working.
In my case memtest86+ 4.20 couldn't be coaxed into realizing it was dealing with ECC RAM; even if I configured it for ECC On, it still reported
ECC: Disabled on the IMC line. I haven't yet tried with a newer version. However (possibly after installing edac-utils, unfortunately I did both essentially at the same time), Linux reports in the boot logs (interspersed with some other entries):
[ 4.867198] EDAC MC: Ver: 2.1.0
[ 4.874374] MCE: In-kernel MCE decoding enabled.
[ 4.875414] AMD64 EDAC driver v3.4.0
[ 4.875438] EDAC amd64: DRAM ECC enabled.
[ 4.875542] EDAC amd64: CS0: Unbuffered DDR3 RAM
[ 4.875545] EDAC amd64: CS1: Unbuffered DDR3 RAM
[ 4.875546] EDAC amd64: CS2: Unbuffered DDR3 RAM
[ 4.875548] EDAC amd64: CS3: Unbuffered DDR3 RAM
which is a pretty good indication. Manually doing
/etc/init.d/edac restart does not create similar log entries, and looking at an older log from a few reboots ago, I see:
[ 13.886688] EDAC MC: Ver: 2.1.0
[ 13.890389] MCE: In-kernel MCE decoding enabled.
[ 13.891082] AMD64 EDAC driver v3.4.0
[ 13.891107] EDAC amd64: DRAM ECC disabled.
[ 13.891116] EDAC amd64: ECC disabled in the BIOS or no ECC capability, module will not load.
[ 13.891117] Either enable ECC checking or force module loading by setting 'ecc_enable_override'.
[ 13.891118] (Note that use of the override may cause unknown side effects.)
dmidecode --type memory also gives two pretty strong indications: the physical memory array's "error correction type" property (which however for some reason showed the same on non-ECC RAM, so this may be related to the motherboard's support rather than the memory's capabilities),
Handle 0x0026, DMI type 16, 23 bytes
Physical Memory Array
Location: System Board Or Motherboard
Use: System Memory
Error Correction Type: Multi-bit ECC
and each memory device's total width and data width, respectively (the additional bits being those used for the ECC):
Handle 0x0028, DMI type 17, 34 bytes
Array Handle: 0x0026
Error Information Handle: Not Provided
Total Width: 72 bits
Data Width: 64 bits