Simple!
1. Ensure you have dmidecode installed. if not install it, for CentOS (yum install dmidecode).
2. Type in commandline dmidecode and it will throw a list information about your hardware recorded in the BIOS for you. Since you want only the ESX Version, you can filter info by issuing “dmidecode -t bios” (without the quote)
3. just look at “Address”
4. You can see from the pic above, I have Address: 0xEA050, compare it on the version mapping below
“0xE8480″ = “ESX 2.5″
“0xE7C70″ = “ESX 3.0″
“0xE7910″ = “ESX 3.5″
“0xE7910″ = “ESX 4″
“0xEA550″ = “ESX 4U1″
“0xEA2E0″ = “ESX 4.1″
“0xE72C0″ = “ESXi 5″
“0xEA0C0″ = “ESXi 5.1″
“0xEA050″ = “ESXi 5.5″
5. so in my case I’m using ESXi 5.5!
That’s it!