Take into account that plants distribution is chaotic although reflecting the ecology of the different species. Thus, if you see some trees or shrubs, especially if ornamental, growing in line it can be rather sure that it is unlikely (not impossible but highly unlikely) that they are there only by chance.
Time does not change anything concerning the possible wild status of a plant. Perhaps you could check if they have produced an offpsring in their proximity.
A rather more difficult case is that of urban gardens where many ornamental trees and shrubs often apparently grow with a random distribution.