Is there a general way of knowing if an elementary boson for example is virtual or real? So for example, two leptons annihilate into an elementary boson. Then you can get real photons, W+-/Z° bosons depending on the leptons. However W+-/Z° and photons can also be virtual when acting as mediators of force. Is this always the case? That whenever it's a force mediator it's going to be virtual and 100% off shell?