decays
In decays, we expect to see two leptons:
end-on view
side-view
Sometimes, in lepton-antilepton Z0 decays, the incoming electron produces a photon. We still observe a back-to back lepton-antilepton pair in the cross-sectional view. However, to conserve momentum the lepton and antilepton will not be back-to-back in the side-view:
end-on view
side-view
Therefore, we need to look at both views to determine whether such an event was a Z0 or W pair decay.
A further complication arises when we consider four jet events. A W pair may decay into two quark-antiquark pairs, but the 
process also produces four jets of particles. (See 'How to Identify Slightly More Complicated Types of Event'.) We then have to look at variables such as the jet energies and the mass of jet pairs, and even then it is extremely difficult to distinguish between the two types of four jet events.