Question

How do you determine pi bonds in a molecule?

Answer 1

look for the double bonds! (or use IHD to calculate this)

Explanation:

pi bonds are only found in double or triple bonds, so any location within a molecule that has these has pi bonds. This includes regions where resonance occurs, including aryl rings (benzene based ring structures).
IHD is the Index of Hydrogen Deficiency. This can be used to calculate how many Hydrogens are "missing" from the formula, if the formula is known. This is based upon the full amount of hydrogens for a saturated compound and therefore assumes that any missing hydrogens must be account for by either pi bonds or ring structures.