Becker v Becker 2016 BCSC 487 nicely summarizes much of the law relating to mental capacity aka testamentary capacity including the law re the burden of proof in mental capacity cases.
51 The burden of proving testamentary capacity is on the party propounding the Will, but there is a presumption of capacity where the Will has been duly executed, with the requisite formalities, after having been read by or to a testator who appeared to understand it. That presumption may be rebutted by evidence of suspicious circumstances, in which case the burden reverts to the propounder to prove testamentary capacity on the balance of probabilities: Vout v. Hay,  2 S.C.R. 876 (S.C.C.).
52 The “suspicious circumstances” must do more than create “a general miasma of suspicion”; they must create “a specific and focused suspicion that the testator may not have known and approved of the contents of the will: Clark v. Nash (1989), 61 D.L.R. (4th) 409 (B.C. C.A.) at 425.
53 Suspicion may relate to circumstances:
i) surrounding the preparation of the will;
ii) tending to call into question the capacity of the testator; or
iii) tending to show that the free will of the testator was overborne by acts of coercion or fraud: Laszlo v. Lawton, 2013 BCSC 305 (B.C. S.C.) at para. 202.
54 The usual civil standard of proof — namely, proof on a balance of probabilities — applies, but as a practical matter the extent of the proof required will be proportionate to the gravity of the suspicion, which will vary with the circumstances peculiar to each case: Vout at para. 24; Laszlo at para. 205.
55 In Laszlo at para. 207, Justice Ballance said there is no fixed checklist of circumstances that will be considered suspicious, but:
 … [c]ommonly occurring themes include where a beneficiary is instrumental in the preparation of the will (especially where the beneficiary stands in a fiduciary position to the testator), or where the will favours “someone who has not previously been the object of [the testator’s] bounty and does not fall within the class of persons testators usually remember in their wills, that is to say their next of kin”: Longmuir v. Holland, 2000 BCCA 53, at para. 69 [Longmuir]; Heron Estate v. Lennox, 2000 BCSC 1553 at para. 67 [Heron Estate].
56 Ballance J. also discussed, at paras. 189 and 190, the question of timing:
 Timing is key. In general, the first relevant time that testators must have testamentary capacity is when they give will instructions; the second is when the will is executed. In recognition of the fact that faltering mental capacity is prone to fluctuate, the authorities permit variation of the degree of capacity required at these pivotal times. For example, the will of a testator who is competent to give instructions, but has lost capacity when the will is executed, may be valid so long as, at the time of execution, the testator was capable of comprehending that she was executing a will drawn in accordance with her previous instructions: Parker v. Felgate (1883), 8 P.D. 171; Brownhill Estate (1986), 72 N.S.R. (2d) 181 (Co. Ct).
 The diminishment of mental capacity, particularly in the elderly, will frequently emerge and worsen over time. In light of that, evidence of symptoms exhibited by a testatrix both before and after the making of the will may support an inference relevant to the determination of the presence or absence of testamentary capacity at the material time: see generally, Smith v. Tebbett (1867), L.R. 1 P. & D. 354at 398; Kri v. Patterson,  O.J. No. 1817 (Ont. Surr. Ct.); Fawson Estate, Re, 2012 NSSC 55 (N.S. S.C.); Moore v. Drummond, 2012 BCSC 1702 (B.C. S.C.) at para. 47 [Moore]; Coleman v. Coleman Estate, 2008 NSSC 396 (N.S. S.C.) [Coleman].