You can measure it against sales if you take into account your expected sales for the period and compare them to the period where the show/movie was shown. In other market research they would have test groups and find out whether people even notice the placement. (Like when coke is placed in a movie for instance)
It would be important to ensure that your brand is placed in a show/film that speaks to your demographic. The Knocked Up placement was killer; you are hitting the right demographic and it's not like someone is holding a Doritos bag for 10seconds, the brand is written right into the script and is prominent. It was likely very effective at getting people to recall the brand name - they could google or type in the site from memory.
Placements are pricey so you'd have to ask someone who's done it if it ultimately pays off.
