How do I know?
A lot of PSAs are really aimed at a different audience than the nominally 'addressed' target. For instance, those TELL CONGRESSMAN XYZ THAT YOU'RE DISGUSTED WITH HR1234
ads are not really aimed at constituents. They're meant to be heard by Congressman XYZ's staff, to give them an uncomfortable feeling that people are disgusted with HR1234.
Recently the local moneytalk station has been running a lot of ads nominally addressed to blind people. The ads are sponsored by a drug company, so presumably the goal is to get blind people to try a drug for circadian problems.
But the phrasing is ambiguous. Is it really for a blind audience, or is it aimed at stirring non-blind folks to contribute to something?
It's for blind people.
How can I be sure? The ad gives a phone number at the end, as most PSAs do. The ad gives the phone number EXACTLY ONCE, without any special emphasis. Ads aimed at sighted folks will always give a number at least 417 times. Blind people don't need 417 repetitions. They're expert at catching and memorizing.