First, be careful before attaching the label “Parental Alienation” (PA) to something which is worrying but not as extreme.  Children of separated parents can find it very hard to manage their divided loyalties, and may show signs that they are rejecting a parent, whether or not they have been influenced by the parent they live with most.

The signs of the far more worrying PA are more extreme, as listed here.  As noted by Dr William Bernet, these symptoms still need to be studied further but do give a strong indication of the disorder.  At the simplest level, building up contact with the child is the best way to prevent alienation.  And if the child is already so hostile that this is impossible, there are now family therapists and child psychologists who have experience of working with children to undo the alienation.  Amongst the mass of information on this topic, try Nick Child’s Alienation Experience, and Karen Woodall’s blog.