What is the burden of establishing that a testator had capacity to execute a will?

The law begins from a presumption that a testator has the requisite presumption of capacity to execute a will. That presumption is rebuttable and may be displaced if it is shown, on a balance of probabilities, that there are suspicious circumstances. If there are, the burden moves to the propounder of the will who must show that the testator had knowledge of and approved of its contents. If the suspicious circumstances relate to mental capacity, then the propounder of the will reassumes the burden of establishing testamentary capacity: Vout v. Hay, 1995 CanLII 105 (SCC), [1995] 2 S.C.R. 876 at paras. 26-27.