What is the test for reasonable grounds to believe in a search warrant?

[37] The concept of "reasonable grounds to believe" imports a measure of uncertainty. In Descôteaux v. Mierzwinski (1982), 1982 CanLII 22 (SCC), 70 C.C.C. (2d) 385, (S.C.C.), Lamer J. (as he then was) stated at p.410: After all, searches, while constituting a means of gathering evidence, are also an investigative tool. It will often be difficult to determine definitively the probative value of a particular thing before the police investigation has been completed. Later, at p.412, he continued: As I have already stated, a search warrant is not only a means of gathering evidence, but also an investigative tool. Therefore, a determination of what is reasonable in each case will take into account the fact that a search makes it possible not only to seize evidence but also to ascertain that it exists, and even sometimes that the crime was in fact committed and by whom.