Preparation:

1. Get two identical phones, one that you use, and one that has a dead battery.

2. Fix a set of product-categories.

Experiment:

Every week,

1. Label the phones A and B. Use a coinflip to decide whether the working one is A or B. Record which it was.

2. Hand the phone to someone else. They exchange the labels for 1 and 2. If heads A=1, if tails A=2, and record the coinflip.

3. Get the phone labelled 1 back. Neither of you know whether it is working or not.

4. Randomly pick a topic from the pre-fixed list and talk about the topic near the phone.

5. Let the other person remove the labels.

6. Pick out the working phone and go about your day.

7. Write down whether you see ads relating to the topic, yes or no.

Analysis:

Join the records, to see which weeks you used a working phone, and if those corresponded to seeing ads.

Good target markets are: Grown adults with $$$ who either don’t have teeth or know someone who doesn’t have teeth (likely parents/grandparents/in-laws). This is basically every upper-middle class adult. More people than you think are missing their teeth.

This is perhaps even more creepy than just "phones listening in", but it's not an explanation I hear very often.

If it's listening, you'll start getting ads specific to that snack food, best to try it with a bunch of different products to try and find a pattern.