Basically, you need to prove (a) that you own copyright in something; and (b) someone has infringed that copyright.
Take a look at the answer to this question on proving ownership of copyright. To prove that copyright has been infringed, it must be shown that (i) the infringer had acess to the copyrighted work; (ii) the infringer produced or reproduced all or a substantial part of that work; and (iii) none of the defences or exceptions put forward by the infringer apply. \”Access\” doesn\’t have to be direct access; it\’s possible to infringe copyright by copying something else made from the original, copyrighted work. Whether the access was direct or indirect, it must have happened; it might not be enough to argue that \”I\’ve published my pattern on the Internet, you have access to the Internet, therefore you must have copied it\” without any more evidence, but it depends on the circumstances.
The issue of whether something is a \”substantial part\” depends on the facts of each case. An infringer might attack the validity of the copyright (perhaps by asserting that your work is not \”original\” and therefore not protectable by copyright) by way of defence.