That's an interesting question. Anybody with a 1986 price list know? The Mac Plus was labeled 1MB on the case back label from 1986 to 1988, when they removed the 1MB. I would certainly expect you could order any configuration up to 4MB from that point on. However, given RAM prices in 1986 and wanting to give dealers the ability to make a profit off the service, they might have shipped only 1MB, requiring a dealer upgrade if a customer wanted more – it certainly would have streamlined manufacture and shipping for Apple. Also, sometime during this period Apple stopped offering mail-orders, everything had to be done through an authorized dealer, so perhaps this figures into it.
In any event, a dealer who opened the case would most likely not show any external signs. A third party might reveal some pry marks on the plastic along the case seams. However, I would be surprised if a Plus was NEVER opened during its entire life. These Macs were highly susceptible to hardware failures of one sort or another. Nevertheless, you can tell from the bottom sides of the inside bucket if a case was repeatedly opened as it tended to make contact with the chassis and rub and scratch the metallic powder coating. If yours is some kind of WunderMac and opened only once to add additional RAM, then there may be no signs at all it was ever opened.