Your filled DIMM slots will show up as elvis showed above; empty slots will show something like this:
root@truenas[~]# dmidecode -t memory
# dmidecode 3.2
# SMBIOS entry point at 0x7f0c7000
Found SMBIOS entry point in EFI, reading table from /dev/mem.
SMBIOS 3.0 present.
Handle 0x0017, DMI type 16, 23 bytes
Physical Memory Array
Location: System Board Or Motherboard
Use: System Memory
Error Correction Type: Single-bit ECC
Maximum Capacity: 128 GB
Error Information Handle: Not Provided
Number Of Devices: 4
Handle 0x0019, DMI type 17, 40 bytes
Memory Device
Array Handle: 0x0017
Error Information Handle: Not Provided
Total Width: 72 bits
Data Width: 64 bits
Size: 16384 MB
Form Factor: DIMM
Set: None
Locator: DIMMA1
Bank Locator: P0_Node0_Channel0_Dimm0
Type: DDR4
Type Detail: Synchronous Registered (Buffered)
Speed: 2400 MT/s
Manufacturer: SK Hynix
Serial Number: 2BCE2302
Asset Tag: DIMMA1_AssetTag (date: 18/10)
Part Number: HMA82GR7AFR8N-UH
Rank: 2
Configured Memory Speed: 2400 MT/s
Minimum Voltage: 1.2 V
Maximum Voltage: 1.2 V
Configured Voltage: 1.2 V
Handle 0x001B, DMI type 17, 40 bytes
Memory Device
Array Handle: 0x001
Error Information Handle: Not Provided
Total Width: Unknown
Data Width: Unknown
Size: No Module Installed
Form Factor: DIMM
Set: None
Locator: DIMMA2
Bank Locator: P0_Node0_Channel0_Dimm1
Type: Unknown
Type Detail: Unknown
Speed: Unknown
Manufacturer: NO DIMM
Serial Number: NO DIMM
Asset Tag: NO DIMM
...
This partial example shows that there are 4 DIMM slots: A1, A2, B1 & B2. DIMMA1 is filled with a 16GB module, but DIMMA2 is empty.
Hope that helps.
Jim