Statutes v. Regulations v. Case Law
The distinction that you make between statute law and case law isn't helpful.
Before a statute is enacted regarding a topic it is governed by the common law, which is a body of law created by courts, address certain legal issues.
A statute, when enacted, can modify the common law rules that apply to a situation. For example, there is not a common law rule that contracts have to be in writing, but the "statute of frauds" which is actually a collection of statutes, overrides that common law rule and provides that certain contracts have to be in writing. A statute can outright overrule prior case law.
Other cases, decided after the statute is enacted, are binding and definitively determinations of what a statute means.
For example, case law interpreting the statute of frauds may state that the statute of frauds is not applicable to cases where there has been part performance of an oral contract.
Sometimes case law interpreting a statute gives it a different meaning than some of its more obvious plain language interpretations, not really overriding the statute, but influencing what the statute means in ways that wouldn't be clear without the case law.
Regulations, like case law, provide another way to interpret the meaning of a statute without overruling it, and can effectively influence how it should be interpreted in ways that wouldn't be clear without the regulation. Case law, in turn, can determine if the regulations were validly enacted and if the regulations are a permissible interpretation of the statute, and can determine what the regulations mean as applied to a particular set of facts.
Can A Layman Determine What The Case Law Says?
There is no direct way that a person on the street can easily determine what case law says through research. They need to rely on lawyers and other secondary sources to determine what the law obligates them to do and what statutes actually mean.
Is there a formula for deciding controlling law?
Yes and no. There are general rules that are considered in determining what is "controlling law", but in practice, applying those general rules is more of an art than a science. Often reasonable people can come to more than one conclusion.
One of the main things that makes the determination of "controlling law" more art than science is the level of generality with which the rules can or should be stated.
For example, there might be a general principle that "you have a duty to pay compensatory damages to someone whom you harm without justification", that is controlling, but there might not be case law determining if the facts that allegedly constitute justification in a particular lawsuit actually constitute justification or not. If there is a case law precedent binding upon the court in which a case is tried with similar facts, courts will try to apply the ruling in that case to your case. But, deciding whether facts are similar or not can be more of an art than a science. It isn't a formula that can be applied by a computer or as an "umpire". The judge or jury has to exercise judgment in applying the law to the facts, and in deciding how specifically the law can be described in a particular situation.
Some rules of law, called "standards" are deliberately vague and call for a fact by fact determination based upon the context of a particular case.
For example, "negligence" is a basis for finding that there is civil liability in a lawsuit, but negligence has to be decided on a case by case basis by the finder of fact without regarding to how juries in similar cases have evaluated similar facts. A jury in one case could find that conduct is negligent while a jury in another case could find that identical conduct is not negligent without the controlling law (that negligence is necessary to find liability) being different between the two cases.
Similarly, the controlling law may be that it is copyright infringement to make a derivative work from copyrighted material whose copyright is still in force, unless it is fair use, and may define all of those terms. But, somebody has to apply the controlling law to the facts to determine if there was a copyright infringement, and the relevant definitions often leave room for interpretation and judgment.
Controlling law means law (both statute law and regulations and case law taken together) that clearly applies to the circumstances presented providing a legal answer. Not infrequently, the answer to the question of "who wins" isn't obvious even when you know the controlling law.
Typically, the term controlling law implies the law of the correct state, as modified by any relevant federal law, and as interpreted by all relevant regulations and case law, where the language of the statute, regulations and case law that apply make it clear that the statute applies to the particular set of facts to which the law is applied.
When there is no "controlling law" because the relevant statutes, regulations and case law are silent on what the law says in a particular circumstance, this is called a "question of first impression."
For example, if a statute says it applies to "livestock", and the term livestock is undefined, and there are no relevant regulations or cases, the question of whether bee colonies count as "livestock" would be a question of first impression upon which there is no controlling law.
But, if a case interpreting the statute determined that a hunting dog isn't "livestock" that case taken together with the statute, would be controlling law holding that hunting dogs are not livestock in future cases.
Resolving Apparent Conflicts Between Sources Of Law
A variety of doctrines govern apparent inconsistencies between different sources of law.
For example, generally speaking, federal law overrides state law under the Supremacy Clause of the U.S. Constitution, but if the federal law takes the form of a treaty, it is necessary to determine if the treaty is "self-executing" or merely imposed a duty on the government to do something without creating a right on the part of an individual litigant to enforce the treaty directly.
If there are cases that reach opposite conclusions on the same point of law, cases decided in courts with appellate authority over the court deciding another case prevail. Only cases decided by courts with appellate authority over the court where the case arises are binding, other cases are persuasive authority but allow the court to reach another conclusion if it thinks that the non-binding precedents are ill reasoned.
There is also considerable effort made to determine if two sources of law that seem to conflict actually do. For example, if there are two contrary definitions of "livestock" in two difference cases, one discussing a tax statute and the other discussing a trespass statute, the definitions determined in the cases apply only to those statutes and don't necessarily apply with the same meaning in other contexts in different statutes or in interpretations of a contract.