What is the test for establishing an estoppel?

"The existence of an estoppel is generally a factual question. [Citation.] Therefore, we review the trial court's ruling in the light most favorable to the judgment and determine whether it is supported by substantial evidence. [Citations.] ' "Substantial evidence" is evidence of ponderable legal significance, evidence that is reasonable, credible and of solid value. [Citation.] . . . [Citations.] . . . [Citation.] Inferences may constitute substantial evidence, but they must be the product of logic and reason. Speculation or conjecture alone is not substantial evidence. [Citations.]' " (Feduniak v. California Coastal Com. (2007) 148 Cal.App.4th 1346, 1360.)