You draw the molecule's Lewis structure and look for double or triple bonds.
You can find pi bonds in molecules that have
- double bonds, which consist of one sigma bond and one pi bond
- triple bonds, which consist of one sigma bond and two pi bonds
The molecule ahs a total of 32 valence electrons, 5 from phosphorus, 6 from oxygen, and 7 from each of the three chlorine atoms.
Phosphorus will be the molecule's central atom. Each of the three chlorine atoms will bond with the phosphorus atom via single bonds, which will account for 6 of the 34 valence electrons.
The chlkorine atoms will each have 3 lone pairs of electrons present, which will account for another 18 valence electrons.
Now for the bond between oxygen and phosphorus. At this point, you still need to account for 10 valence electrons.
If you use a single bond to connect the two atoms, the remaining 8 valence electrons will have to be placed on the oxygen atom. This will give the oxygen atom a -1 formal charge and the phosphorus atom a +1 formal charge.
A much more stable configuration is the one where the oxygen atom has 2 lone pairs of electrons present, and the bond between itself and the phosphorus atom is actually a double bond.
Here's how the Lewis structure looks like without the lone pairs shown