A contract is not a piece of paper; it is an agreement intended to be legally binding between 2 or more people and it may be verbal or written or a combination of both.
That said, where a person has signed a document knowing that it contains contractual terms, in the absence of fraud that person is bound by the terms: it is immaterial whether the person signing reads the document or not.
So there are two reasons why a signed contract would not be binding:
- the person did not know it contained contractual terms
It is in the second case that the validity of the signature would matter. Fraud would have to be proved: it would not be sufficient to say "I did not sign that"; the person would need to demonstrate that a fraud has been perpetrated.
Particularly today, with the ability to scan a signature it is trivially easy to affix anyone's signature to anything.
However, a party to a contract is entitled to rely prima facie on the validity of the signature. A person would have to provide evidence that it was not their signature or had been affixed without their knowledge or consent.
A court would look at the entire circumstances surrounding such a claim; if a person had, up until the dispute, acted as though they had signed the document then a court would probably not countenance an argument that they hadn't.
It is always possible to construct contrived circumstances where this or that could happen but, in reality, they are extremely rare. Unless you are dealing with a con-artist, you can trust the signature; if you are dealing with a con-artist, you have bigger problems.
See, the President has endorsed this answer: