From the 'Fringe' wiki "Fringepedia":
According to the episode "August", The Observers (though it is not made clear which observer specifically) have been spotted at various important events throughout history evidenced by his appearance in notable historical paintings and images.
An Observer is present as an onlooker in the crowd depicted in the painting "The Bloody Massacre" which depicts the Boston Massacre of 1770.
An Observer is present, again as an onlooker in a crowd in a painting of Marie Antoinette being taken for execution during the French Revolution
An Observer is present in a photograph of Archduke Franz Ferdinand and his wife leaving a building moments before their assassination which served as the catalyst for World War I.
"It's more like they're observing time. I think these guys show up at important moments. Um, historical, technological, scientific. But it's rare. Maybe two dozen different definite occurrences in the past 5,000 years of human history. But recently, I've charted twenty-six sightings in the past three months alone. It kind of makes you wonder." - (Lab technician Brandon, to Olivia and Peter in "August" about the research and data he had collected)
Since the Observers from 'Fringe' have been around for a few centuries (at least), were they ever able to detect the presence of the Silence? It really wouldn't matter if they could - their function seems to be just to observe.....