The Smarm Offensive

I’m confused. Am I to assume all sincerity has an ulterior motive then? How can I tell when someone is actually being earnest?
It seems like it depends on the mood you’re in when dealing with the individual. Its easier to be cynical and dismissive when faced with earnestness, rather than consider the possibility that the individual may be sincere. Even if they are being smarmy, being so on-guard all the time will make you come off as just another gloomy pessimist.

Do you just challenge them to provide substance? How much substance is necessary for the person to provide before they pass smarm litmus testing?