It's obivous when someone is doing a commercial on the radio. With the exception of Paul Harvey (but if you're a fan, you'd know quite easily that the ads are on Page 2 and Page 4 of his 5 page newscast at noon), there is a very clear line of demarcation - they usually stop the show altogether, and the "live read" commercials are more often than not, prerecorded.
There is a big difference between a paid endorsement running in a commercial and a government buying off a critic.