How to know if a particle is on shell in general?

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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?
 
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There is no "100% off shell". If you know enough momenta of involved particles (e. g. all decay products), you can calculate the 4-vector of the intermediate particle, and see if it was (approximately) on-shell or not.
 
Technically, since these gauge bosons are unstable (and what you actually detect are jets/leptons) all gauge bosons are off shell.
 

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