Okay here's the answer. What you are talking about doesn't have a name yet. There a many different types of intelligence and an IQ test only tests some of them. So you are right in the sense you can't really tell how "smart" someone is off of that. For the purposes of this discussion we will now refer to the word that doesn't really exist yet as "smart or smartness". How do you know if someone is "smarter" than you? That's subjective all on it's own. I think you meant to say, how are the "smartnesses"(i know oh god bad english but i had to say it that way) of any two people different and either what process is behind our ability to "tell" if someone is smart or you want to know how can someone who is less smart than someone can tell? Well, if there is a big enough difference in IQ, most people can tell just by talking to a person. A long time ago I heard that if there is a difference of 30 IQ points then it becomes difficult to understand a person because of the difference in known vocab and daily use of that vocab. But there are many different ways of seeing if someone is smart or not.
That's boring so let's talk about why everyone is wrong about intelligence. I think you are more after that then anything else. So know that the only reason we have an IQ test, is to test for the difference in the standard deviations between the "intelligence" of children to see if they need help or advancement. 2 SDs below or above makes you different from everyone else and you help special needs in some aspect. That's an extremely crude way of explaining that all, but it works for the purposes of what I am trying to explain now. (That's 30 IQ points up or down from 100 by the way). For the most part, IQ tests for adults don't really tell us what we are trying to find out, they were designed for children. If you want to see if someone is actually intelligent across all aspects of adult "smartnessess", you can't use an IQ test. Think of the IQ test being used for adults as a way of measuring how well they may do in school or a similar area. But if we want to really find out how "smart" adults are we do need all aspects of intelligence that we currently know. For example, ever heard of street smarts? Some people claim they have street smarts and they don't need a formal edu because of this and that's what makes them "smart" or smarter then everyone else. Street smarts can mean a million different things but lets assume it means skills that have been picked up only by "living" on the streets, AKA a low econ area with high crime or moderate crime in most cases. Not that you are self taught in all aspects of intelligence. So while "street smarts" could be an aspect of intelligence, not the full picture. Also someone with street smarts might just mean they are self taught in all aspect and they very well could be our definitions of smart. Remember that school is designed for the average person and it is not suited for people that stray too far from the average(in any aspect of "smartness" but in the case of school we can say IQ). A smart person will be smart regardless of a formal edu or not and so will a intellectually challenged or disabled person will always be that way(brain damage in their case doesn't go away). As long as the damage is physical and not psychological. Anyways, I personally don't think we have mapped out everything yet as far as our true intelligence goes because for the simple reason of we just haven't gotten to it yet. Unless you have an idea on how knowing your real adult IQ could help anyone besides helping find people with intellectual disabilities which need help(which the current test can already do), there is no application for it. Also it's been proven in the past that it's harmful for people to know how "smart" they really are.(look up army test alpha and beta i think, something like that), everyone overestimates their "smartness:. I personally think it would be harmful to the public if they had that info. In the US that is. I dunno about other countries but they are probably the same in first world countries. I doubt anyone who has to worry about survival, worries about how smart they are. Well I hope that answered your question. If you want sources, I am lazy and there is enough info for you to check any claim i've made on google. You wanted the answer anyways so you do the leg work if you don't believe me. There's also my opinion but i;ve stated which of this is my opinion. The rest is from about 7 years of college in psychology and another 3 years or so of personal research in the area of psych. So...I could be an expert just based on all the hours I've spent in the field of psych. So the opinion of an expert in his field is probably good for his word on some of his opinions yes?
I mean no malicious in anything I say, I am just normally sarcastic, so no offense on anything if you have taken it that way. I meant none.