1. Open a web browser that is on a system local to the vmhost.
2. Browse to: https://<<HOST IP>>/mob/?moid=ha-host&doPath=capability
3. After you login, search for the nestedHVSupported property on the page and you should see a value of either true or false.
If it is false, you might still be able to virtualize systems but you will not be able to run nested 64-bit virtual machines. Take a look at your system BIOS to ensure things like Intel-VT/EPT and AMD-V/RVI are enabled.