If the price of product A drops and the demand curve of product B shifts right, then it can easily be concluded that products A and B are complementary products.
When a curve shifts right, then there is less demand for it when another product is at a certain price point. If product A is just as good as product B and costs less, then more people are going to buy product A. Product B’s demand will shift right on a demand curve.
If product A’s price went up, then the curve for B would shift left instead of right. Especially if the two products are more or less the same and are complementary goods. That’s all it means, really.