But how do I ensure that my credences prior to this verification are accurate? As in, how do I know if it is rational for me to have a higher credence in my partner cheating on me vs. there being a water bottle in my fridge.
Let's suss out an important distinction between empirical methods and rational methods. Hiring a PI to follow your spouse and checking the contents of the fridge are empirical methods because they rely on empirical evidence:
Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessible to sense experience or experimental procedure. Empirical evidence is of central importance to the sciences and plays a role in various other fields, like epistemology and law.
Fundamentally, you know that your beliefs can be evaluated to a truth-condition by using your senses, relying on facts, third-party testimony, etc. This might be considered a posteriori. On the other hand, what you are asking, when you use the phrase "rational for me" is not a question of the empirical, the sensory, or a posteriori, so much as the using reason before testing your hypothesis or determining the truth condition as a function of the state of affairs in the world. One can use reason, but do so poorly and so if one does so well, it might be considered being rational. Reason and rationality are terms that have a number of sense, so it's important to see that there is the use of reason, but there is also the reasonable use of reason. Rationality tends to capture the essence of the latter. To test your credences before determing their truth-condition, consider whether or not they are rational. An example might help to make this distinction concrete.
I have encountered what I call fallacy mongers. People in arguments who know the name of fallacies, and attack your position by declaring every other claim fallacious. For instance, one can reasonable cite Einstein as an authority in physics and matters of physics given that he helped to craft a historically and philosophically important theory in regards to space-time in distinction to Aristotle or Newton before him. Yet, a fallacy monger might accuse you of appeal to authority to undermine your point not understanding that there are criteria that allow you to differentiate between appropriate invocation of authority and inappropriate conditions. The fallacy occurs ONLY in the latter condition. No matter to the fallacy monger! Any mention of authority is irrational (more so when it undermines their point when they claim they have found an exception to the conservation of momentum in their garage which is a true anecdote of mine). Thus, they are being reasonable using the language of reason, but doing it poorly or unreasonably.
- The first thing to learn about when being rational is what philosophers have to say about rationality itself. Consider reading the Oxford Handbook on Rationality (GB). It might not instantly make you more rational, but at least you'll have some ideas about what epistemologists and professional philosophers believe and articulate rationality to be. People tend to believe they are more rational then they actual are.
- Read up on fallacies, cognitive biases, and the philosophy of logic. There are books on all of these topics. Again, it is part of the hubris of the human mind to overestimate it's capacities in these topics once a little information is acquired. Dunning and Kruger are famous for showing that there's a difference between having ability and having estimates of ability, and that without lots of talent and/or hard work, the latter tends to be optimistic. There are people on this site who have obviously spent their lives mastering logic, and there are people on this site who argue with them obviously armed with a flimsy understanding of the basics. Don't be the latter; exercise humility.
- Read philosophy. Start off with encyclopedia entries and primary sources. Have you read Kant's Introduction to Logic? Descartes Meditations? Good. Now read professional philosophers interpretations of them in encyclopedia entries and articles and books. Do you understand the historical movements of empiricism and rationalism? Descartes, Hume, and those who follow often have profound insights into reasoning.
- Read psychology. Cognitive science, behavioral economics, and psychological linguistics have profound ideas, hypothesis, and ontologies. Make use of what science says. This is the notion of a naturalistic epistemology. Many "philosophers" like to believe their brain is an logical engine on par with a supercomputer, and that their introspection is comprehensive, rational self-knowledge. Nonsense. Our brains and bodies are highly imperfect, and our knowledge largely is fallible (IEP) and intuitive (SEP).
And prepare for the long-haul. Being rational and reasonable, particularly in the face of the tempestuous nature of life might call for epoche and ataraxia, and other approaches to life such as acknowledging that work is opportunity. In life, you'll have a window of opportunity from the time your brain fully develops in your early twenties to a possible cognitive decline in old age. Being rational isn't a course or book, it's a lifestyle choice, one that most philosophers, professional or otherwise aspire to.