You don’t. I don’t place an ad on the side of a random bus, instead I place it on the side of a bus that drives a route and schedule I know. If I buy a newspap

You don’t. I don’t place an ad on the side of a random bus, instead I place it on the side of a bus that drives a route and schedule I know. If I buy a newspaper ad I don’t buy unless the paper is well known enough that I can trust their number they claim is their total circulation. Yes: for the web this means no one buys ads on the bottom 99% of sites.