I have a rule of thumb. It only applies sometimes. When? How can I tell?
I have another rule of thumb. Sometimes it conflicts with the first. When? What to do then?
I have some rules of thumb to answer these questions. They're only heuristics, so they don't always apply. When? I need more heuristics!
Unless we have some way to Normalize Heuristics, they tend to grow combinatorially ...
Indeed, on this wiki, "heuristic" doesn't mean "rule of thumb". But we can still explore the difficulty of how to apply rules of thumb that don't apply in all circumstances. That's a heuristic problem!
The short answer:
If "How can I tell?" means "What is a completely systematic and infallible rule by which I can tell?", there are no such things that apply in real life. Ultimately, all you can do is guess.
The medium-length answer:
Systematic, rule-based judgement is a tiny sliver of your full powers of cognition, and it requires non-rule-based judgement to tell when and where it applies. For example, how can you tell that your cat is hungry? By the way he's purring and walking back and forth across the keyboard even as you keep trying to push him out of the way? That's probably part of how you can tell, but really there are a trillion factors going into that judgement, most of which you can't possibly be conscious of. And even if you identified the full rule that you're applying (assuming such a thing even exists), what rule could tell you that that rule is valid in this case?
If you held back on making judgements like "my cat is hungry" until you had a complete set of rules, which themselves had been validated by other rules, all the way to perfect bedrock rules that are rationally unchallengeable, your cat would starve before you did something. Yet, your brain has astounding powers to make guesses of all sorts, based on negligible amounts of information. You're typing a wiki page, you sense this annoyance at the corner of your consciousness, and suddenly the amazing machinery of your brain puts a guess into your mind: "Hey, I'll bet Rufus is hungry." So then you place your bet and pour some food for him. The bet is not a sure thing, but nothing is.
The long answer:
In many cases, a rule is actually a sort of heuristic, which makes very definite assumptions about the Search Space you're presented with. For example, a rule like "save 10% of your income for a rainy day" assumes that you live in a world where rainy days occur at unpredictable times and where saved-up stuff can last a long time. It's a heuristic that searches for an efficient way to spend your money, so you're in a position to enjoy the good times and get through the bad times when they come.
Every heuristic makes assumptions about the Search Space where you apply it. If those assumptions are false, the heuristic won't work or will work inefficiently. Your question really is, "How can I know for sure that my Search Space really has the properties assumed by my heuristic?" And the answer is: you can't. All you can do is guess.
But it's possible to refine your ability to make good guesses. One of many ways to do that is to become consciously aware of real Search Spaces and their properties. For example, "save money for a rainy day" assumes that the currency will remain stable for many years. There is no way to know if the currency will remain stable for many years. Now that you're aware of that, you don't have any clear-cut definite rule to follow to ensure that you're always prepared for rainy days. But your ability to make guesses about this topic is slightly more refined. Most importantly, your ability to recognize and make use of unanticipatable information is now improved. What is that information and how will you interpret it when it arrives? Right now, you can't know.
Ultimately, the most powerful heuristics are those that lead you to knowledge of a sort that you can't anticipate. Fully systematic rules are ways to get things that you can define in advance, in environments where every possible observation is known, along with the appropriate response, so all that's left is to make the observation and then perform the response. Such perfectly understood regions of reality are only a tiny part of what's there, though. Most of the goodies live in the Yawning Void.
A very topical area where we use heuristics is in the identification of SPAM emails. How do you know an email is SPAM? There are so many different sorts of SPAM emails the Search Space is very large, and it also changes with time. It would be impossible to write a comprehensive definition of a SPAM email. Therefore we often apply heuristics to delete such emails. One heuristic might be "do I know the sender?" If the answer is no, we delete the email. The email may well have been legitimate, but in order to eliminate the large number of non-legitimate emails quickly we apply the heuristic (rule) anyway.
When we use judgement, we might say to ourselves "I should delete this email because I dont know the sender, however I am expecting an email from my bank and this might be it". Judgement should always be used when applying heuristics.
See also: Judgement Is Easy.
Cognitive Biases:
Social Psychology shows that in practice Heuristics have some troubling obstacles to overcome. The most apparent obstacle is its relationship to creating bias. Cognitive Bias can be 'harmful'. For example a recent study shows bartenders serve more alcohol in short&wide glasses versus tall&skinny.(http://www.cbsnews.com/stories/2005/12/23/health/main1160885.shtml) This is a human idiosyncrasy and cognitive bias related to the heuristic of anchoring and adjustment. So a woman might feel and be drunk but because she "only" had two martinis she drives home. Impaired judgment and human error have in this example led to a potentially harmful situation.
Yet in all situations humans are given to a natural 'impaired judgment' because of cognitive bias. It might be argued then that humans live in a persistent state similar to that of the drunk woman, impaired and unaware. Sociologist therefore examine a society functioning with the determination to overcome its inherited defects. Does the salvation of the human race lie in the hands of social order? Probably so.
Unfortunately heuristics is relative to the greatest of social atrocities known. The contagion heuristic or 'group think' has driven normal individuals to do irrational even psychotic things. This mob mentality drives crowds/armies to rape, murder, and plunder. Its also found in group decisions like the recent errors that the US Senate Intellegence Committee admits were made about Iraq's weapons leading to the war.
The obstacles in heuristics are like SPAM emails. Society, like the internet, has many threatening viruses ready and waiting. If one clicks on all the popups/groupthink he will find himself looking at cartoon porn/social atrocities. Heuristics are short cuts in discovery which means they can lead to great inventions but also great tragedy.(johnowensemail@yahoo.com)
One may wish to note that the preceding discourse makes heavy use of a heuristic in order to make an argument: namely analogy.