Had en interesting problem where a ESXi host only showed it had 30GB of memory, but the motherboard was populated with 6*8GB modules. In earlier versions of ESXi 5.5< it was possible to use dmidecode to show how the physical hardware was populated. But since 6.0> that have been removed.
The new command to find those kind of information are now “smbiosDump”
smbiosDump | grep -A 6 'Memory Device'
You can also just run smbiosDump without any paramenters and you get a hole lot of information to crawl through.