1. A method comprising: determining whether a user, who utilizes a computing device to interact with a computerized service, (A) is a user interacting with a non-virtualized computing device, or (B) is a Virtual Machine (VM) running on top of a Virtual Machine Monitor (VMM); wherein the determining comprises: monitoring response of the computing device to an interference that was introduced to a communication session between the computerized service and the computing device; based on the monitored response, determining whether said user, who utilizes the computing device to interact with a computerized service, (A) is a user interacting with a non-virtualized computing device, or (B) is a Virtual Machine (VM) running on top of a Virtual Machine Monitor (VMM).