What has to be proved
A forgery prosecution requires a false document (made, signed, sealed or altered dishonestly so that it purports to be what it is not) together with the intent to cause damage or injury, to support a claim or title, or to commit fraud.
Merely being wrong is not forgery. An error in a document, or a dispute about what was agreed, is not the offence. The dishonest making of a document that lies about its own origin is.
Where forgery allegations actually arise
In our practice they cluster in a few recognisable places.
Proof: why the expert evidence decides it
Handwriting and document examination is usually determinative. Specimen and admitted signatures are compared with the disputed one; ink, paper, impressions and sequencing may all be examined. Courts can direct specimens to be taken for comparison.
Practical advice for anyone alleging forgery: secure the original document, avoid handling or annotating it, and resist the temptation to obtain an informal private opinion first, the chain of custody matters, and a compromised original weakens even a strong case.