1. A computer-implemented method for an application to verify presence of an individual interacting with a handheld device versus an automated program executing on the handheld device, the computer-implemented method comprising: prior to presenting any explicit presence challenge on the handheld device: receiving a request from the handheld device; subsequent to receiving the request, collecting, based on an implicit presence test and from one or more sensors of the handheld device, implicit presence data characterizing a movement or position of the handheld device; and upon determining, based on the implicit presence data and by operation of one or more computer processors when executing the application, presence of an individual interacting with the handheld device, approving the request; wherein the application is configured to, upon determining, based on the implicit presence data, that presence of the individual interacting with the handheld device is inconclusive, present an explicit presence challenge on the handheld device in order to verify presence of the individual interacting with the handheld device, including the application being configured to: collect, from at least one sensor of the handheld device, explicit presence data responsive to the explicit presence challenge; upon determining, based on the explicit presence data, presence of the individual interacting with the handheld device, approve the request; and upon determining, based on the explicit presence data, absence of any individual interacting with the handheld device, deny the request as having been submitted by an automated program; wherein at least one of the implicit presence test and the explicit presence challenge is selected based on sensor availability and one or more sensor properties.