RHEL - How to Tell a Server is Virtual

There's a Linux system command that reports hardware specs at /usr/sbin/dmidecode

This command has massive output about every single thing in your system, down to each individual memory module, you want to look for the section "System Information".

The piped command grep -A8 tells the shell to look for the line "System Information" plus 8 lines below.

# /usr/sbin/dmidecode | grep -A8 'System Information'
System Information
    Manufacturer: Red Hat
    Product Name: KVM
    Version: Not Specified
    Serial Number: Not Specified
    UUID: 40928289-EDF3-450E-19C5-B3A5DDE020F9
    Wake-up Type: Power Switch
    SKU Number: Not Specified
    Family: Red Hat Enterprise Linux

The line you're looking for is "Manufacturer". 

In my case above, it's a KVM virtual server running out of a Redhat Host.
In case of VMware virtual servers, Manufacturer will show as "VMware, Inc."

In case of physical server, the Manufacturer line will show known hardware vendors such as IBM, Dell, HP, Sun, etc...