How to prove a breach of contract?

To prove a breach of contract you establish four elements: a valid contract existed, you performed your side, the other party failed a material obligation, and that failure caused you measurable damage.

The standard of proof is the civil one, preponderance of the evidence, meaning more likely than not. Not the criminal beyond reasonable doubt.

How hard it is depends almost entirely on your paper. A signed agreement plus invoices, delivery records, and the email thread showing what went wrong makes the case straightforward. A verbal deal makes every element a fight, starting with whether a contract existed at all. That asymmetry is the strongest practical argument for written agreements: you are assembling your evidence before any dispute exists, for free.

A signed client agreement is the first and heaviest piece of that evidence.