Parental alienation is often described in legal and clinical writing as a pattern where a child is pressured, influenced, or aligned against one parent in ways that go beyond a normal conflict. The Canadian Justice Department’s HELP Toolkit notes that rejection of a parent by a child can involve multiple contributing factors, including a child’s own experiences, the other parent’s behavior, and the broader family context, so it is important not to jump to conclusions from one surface sign alone. Research also links parental alienating behaviors with significant mental health harm for children, including anxiety, distress, loyalty conflicts, and damaged relationships with the rejected parent.

Source and more information: https://www.justice.gc.ca/eng/fl-df/help-aide/tab15-onglet15.html

Signs that are commonly discussed in articles and professional resources include a child repeating adult language that sounds rehearsed, showing a sudden and intense rejection of one parent without a proportionate reason, seeming fearful about expressing affection for the other parent, or mirroring accusations that seem beyond the child’s age or direct experience. Articles on alienation also describe children who appear to be “choosing sides,” feel pressure to protect one parent’s feelings, or change their story depending on which parent is present. In serious cases, the child may seem to be managing the relationship like a performance, saying what feels safest rather than what is fully true, especially if there are threats, punishments, or emotional consequences tied to honesty.
Source: https://righttolove.uk/diagnostic-signs-a-child-has-been-manipulated-to-reject-a-parent/

The best practice is to look deeper than the surface and ask careful, open-ended questions in a low-pressure setting, because a child’s words can reflect coaching, fear, or loyalty binds rather than their whole reality.

That is why professionals often recommend examining patterns over time, the child’s behavior across settings, and the presence of any intimidation or manipulation, rather than relying on a single statement.
Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC9026878/