First of all, physical machines tend to have more memory than VPSs. Question 512MB or less. Secondly you can check several things to find a VPS. You'll commonly find virtual machines have surprisingly basic looking hardware in them. Like KVM has a "Cirrus Logic GD 5446" graphics card. VMWare used to have an RTL8129 network card in. This is so most OS installation media has drivers for the virtual devices.
The facter (part of Puppet) virtual.rb script has several useful techniques for finding out what type of machine you're running.
Look for one of
VMWare or Parallels
lspci and look for VMWare VGA adapter
dmidecode and look for mention of VMWare or Parallels
Run lspci and look for
RAM memory: Qumranet, Inc. Virtio memory balloon