The law is not static. Cases may be overturned on appeal. Judges may disagree with established precedents. Social norms change over time.
To find out whether a case is considered to be "good law" -- reflecting current practice -- you need to check whether subsequent decisions have followed its precedent or not. This is called 'noting up a case'.
"Good law [is] a term used to indicate that a statute, regulation, or judicial decision is current and still applicable. ... A judicial decision cannot be good law if it has been overruled or reversed, or if the statute on which it was based has been changed."
Source: Glossary of Legal Research Terms by Diane Murley, Southern Illinois University Library, 2006. [Archived at: http://fliphtml5.com/hxav/awrf/basic]
Legal databases such as LawSource, CriminalSource and CanLII include tools for noting up cases.
The noting up tool in LawSource and CriminalSource is called KeyCite. It is more powerful than CanLII's tool because it indicates HOW subsequent decisions have treated each case that they cite (e.g. reversed, followed, etc.).
Video source: WestlawNext Canada at CLBC: An Overview - Webinar Recording from Courthouse Libraries BC. The segment on noting up starts at the 45 minute timestamp.
CanLII's tool for noting up is called Reflex. It is not as powerful as the KeyCite tool in LawSource and CriminalSource because it simply lists cases which have cited a specific case. It does not indicate HOW they were treated (e.g. reversed, followed, etc.).