DNA evidence can be useful to both sides in a criminal case -- either to convict a guilty person or acquit an innocent person. In this respect, DNA evidence is no different from any other form of evidence. Both sides are free to argue over how much weight, if any, the jury should give to the evidence in deciding guilt or innocence.
But let's take your specific question. You would prove such a thing through the usual methods -- testimony, documents, exhibits, and the like. In order to convict, the jury must be convinced beyond a reasonable doubt, after considering all of the evidence introduced by both sides on the question, that the DNA evidence was not planted.