Summary description of research project involving a simple technique that can be used to detect virtual machines (VMs) that reside on the same physical host. Detection of co-resident VMs is one possible goal for an attacker conducting reconnaissance. If detected, an attacker could then attempt to gain access to VM memory andor other VM resources and possibly steal data or inject malware. Since cloud-based services utilize virtual infrastructures the ability to detect co-resident VMs has important implications for security in a virtual machine environment.