Differentiation of late diastolic ventricular ectopic from intermittent pre-excitation

How can you differentiate a late diastolic ventricular ectopic from intermittent pre-excitation?

Differentiation of late diastolic ventricular ectopic from intermittent pre-excitation:
The PJ interval (onset of P wave to end of QRS complex or J point) will be constant in WPW syndrome while it can vary in late diastolic ventricular ectopic beats.