Assuming the pants in question are within 10 feet of the caster (20 if a sorcerer is using the distance metamagic on Prestidigitation for some reason) and the caster is targeting only a cubic foot region strategically positioned to embarrass the victim.
Is it possible to target something worn/held by an unwilling creature with Prestidigitation?
If so then does this provoke a saving throw/attack roll or does it happen automatically?