I dont have a clear answer, but here are some random points related to the question:
A merchant would often prefer a knarr over a longship, since it can carry more cargo. Knarr would be shorter and wider than a longship.
Technically, you can see from a distance if a ship is heavely loaded or without cargo by its movements and how deeply it lies on the sea.
There was not necessarily a class of traders amongst Norse. A trader would carry arms and be capable of raiding, and a band of warriors would have to sell their loot from their latest raiding.
Trading and raiding would not necessarely occur on the same location.
Trading was organised. A merchant would go to a known buyer with whom he had a strong personal relationship, even a family connection through a marriage alliance. Or he would go to a known trading port (presumable well defended) or an assembly. In that place, the Viking ship would be a familiar sight rather than a cause of worry unless something was unusual.
Raiding from a small party would rather occur in a more random place, far from good defenses or known friends or allies. (There would be exceptions to this ofc.) Seeing a viking ship in that place would probably be unusual.
A big raid on a major city/trading post would mean an armada of ships rather than a lone ship. That would probably smell of an attack as soon as the first ships are noticed.
Vikings/Norse were not the only raiders in that era, and the raiding is less important to their culture and society than portrayed in popular culture.
On the other hand, Vikings would raid other vikings as well. The Norse settlement in Dublin for example was attacked by other Scandinavians in a well organised raid.
Your question runs along a binary that historians have begun to tease apart in recent years: what is the difference between a trader and a raider? This distinction may have not been meaningful to people at the time.
Looking at the annals of early medieval monks along the North Sea, the initial image of the northmen—as the Irish latin texts refer to them—was that of barbaric, uncivilized warriors. The Irish are particularly harsh toward them, but my favorite is from Constantinople in 860, when some Rus attacked the city. In the wake of the assault Photius, a witness and Byzantine historian, wrote:
"Infants were torn away from breast and milk and life itself and their bodies were dashed against the rocks which became their graves. Their mothers were slaughtered and thrown upon the still convulsing bodies of their infants… Nor did their savagery stop with human beings but was extended to dumb animals—oxen, horses, fowl, and others. There lay an ox and a man by its side, a child and a horse found a common grave, women and fowl stained each other with their blood. Everywhere dead bodies."
This horrific description echoes what we find in other places who suffered at the hands of raiders, describing them, in short, as a people bent on little more than destruction and ruin. But, it is best to understand these statements as works of hyperbole by those who had recently been subjected to a traumatic event. Think, for a contemporary example, of the crazy descriptions given by police officers in the wake of a violent act.
Likely, when a settlement saw a ship on the horizon, there was a certain level of uncertainty: was this ship friend or foe? The answer was probably both. On the ship, simultaneously, a decision would have to be made: what role would they play here? A calculus that we don't fully understand would then be worked out. A group of northmen who had just raided an abbey down the river, might seek to trade their booty for relevant items at the next town they encountered, and so on.
I'm not a historian of the Vikings, so I welcome those who are to contribute/critique. I think the best work about Viking-age culture in Britain and the larger the North Sea is coming from Robin Flemming and her cadre of PhD students at Boston College. Take a look at their work, and her book, Britain After Rome (2011).