You can't ask people. Very few people will admit "I pirated this because I didn't feel like paying for it like I normally would." People always find ways to justify their actions, so every response would just be some variation of "I pirated this because the legal options are too inconvenient that I normally wouldn't bother so I didn't hurt anyone."

And you never will be able to totally eliminate piracy nor will you be able to definitively attribute gains or losses to changes in piracy. Maybe the quality of the content changed, maybe the economy had a downturn, maybe an advertising campaign was really effective, etc.