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: generating and introducing an interference into a communication session between the computerized service and the computing device; monitoring response of the computing device to said interference; 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).