What is the test for finding that a spouse exerted undue influence over a deceased spouse?

"'Direct evidence as to undue influence is rarely obtainable and hence a court or jury must determine the issue of undue influence by inferences drawn from all the facts and circumstances.' [Citations.] Thus, . . . the pressure, or undue influence, may be established by circumstantial evidence. [Citation.]" (Lintz v. Lintz (2014) 222 Cal.App.4th 1346, 1355 [affirming trial court judgment voiding decedent's trusts based on finding that spouse exercised undue influence over decedent].)