While it is not entirely accurate, simply rubbing the pearls on your teeth will give a clue-
real pearls feel slightly gritty, while imitations are often smooth.
The problem with pearls of this age is that real pearls then were very expensive and parents - wanting to give a classic and lovely present for a 21st birthday or a wedding, often scraped together enough to buy some of the imitations which were then expensive and available - Ciro and Majorcan are two types. They probably cost more than a very good real pearl necklace does now, and were rightly treasured by the recipient. Some of the imitation pearls have more of a tendency to yellow over the years too, while real pearls will stay white/ivory