Outdoors the easiest way to validate PP is on short rolling hills where you can charge into the hill, stand up, and hammer until you bleed out. 
A few bursts should do it. (5-7 sec minimum)
Indoors you can do the same in Slope mode or select the standard fitness test and stand up and stomp for the two sprint intervals after warm-up.
You likely won’t hit the same wattage indoors but here’s a related topic –
Is PP a theoretical number? - General - Xert Community Forum