That depends on which type of VOR receiver an aircraft is using. Smaller aircraft typically use an OBI (Omni-Bearing Indicator) which allows a pilot to tune into the VOR signal and get a fix on it, and navigate according to the clarity and volume of that signal. If the pilot begins to go off course, the signal will begin to fade and if it does it means that the pilot is drifting off course. The only way to stay in course with an OBI is by making sure that the VOR signal remains loud and clear.
An RMI (Radio Magnetic Indicator) presents the VOR signal to the pilot on a rotating card and the plane as a triangle on said card. By adjusting the triangle’s position on the bearing presented on the rotating card (which is shown on screen), the pilot can adjust the course of the plane being flown and lock on to the signal and bearing that the VOR is sending.
HSI (Horizontal Signal Indicator) and RNAV (Area Navigation) systems work in conjunction with the onboard computer of an airplane to present the pilot with the data that the VOR beacon is sending in a much more user friendly and easy to understand way. Pilots can then use that information to stay on the beacon trajectory, or if they need to, to adjust the course of the airplane in order to correctly follow the information that it provides.