OneToOne relation is much more constrained than ManyToOne.
So if there is no back reference - it is more likely this is ManyToOne relation.
In the case of back reference existence - ManyToOne will always be a safe bet,
so in this ambiguous case ManyToOne seems is a better choice -
but this is thing for discussion - there are some questions here.
+ may be we should add some comments into sources.