Stand Alone or VM

I've just inherited a mess of a network, but fortunately, some assets are on VMs.  Access to the machines is via VNC, RDP, SSH and/or console.  What I am wondering is, is there any way to tell once you are in as admin on a Windows server whether or not this is a VM or a "normal" non-virtualized server?  How about Linux?  What could I run on a Linux server to know that I was in VM, as opposed to a normal install?

Thanks for any help!