An extreme example of what I have seen is a child who obviously never had any rules set for him. When we visited he, maybe aged four, kept wanting to get the attention of his parents and did this in very extreme fashion which included throwing his food on the floor (the mother quietly picked it up, saying he shouldn't do that), climbing onto the furniture including a bookcase (the mother giggled), keeping up constant shouting and screaming (we all raised our voices in conversation), etc., etc. Other less obvious examples include two children in a family who at every meal time were asked what they wanted and the mother prepared separate meals for each and then something different for the parents after that. To me examples like this are those of spoiling children but, clearly, the parents did not appear to see anything untoward. I have not seen any of the children I mentioned for a decade or more and the latter example will now be adults - they may be quite well adjusted now, for all I know.
I have got to know adults who show signs of having been (and even by their own admission were) "favourites" or indulged as children. One of these got heavily involved with drugs and blames her parents (for spoiling her), although I feel that is not a valid excuse - she actually got herself out of all that through her own strength, which sort of proves that she was also the prime/only real initiator (she could have avoided it, she has been clean for about two decades). To me, in at least two of these adults the main sign of having been spoilt comes in the form of being particularly poorly prepared for adversity, the reaction is something of that of a drama queen. A trait they have in common what to me is a near obsession with their health. They both look for "magic" solutions to ensuring good health or improving it, in a passionate conviction similar to how some people won't step on joints between or cracks in paving slabs - they constantly hop (after some months) from one fad/discovery to another, although one of them is the above ex-addict who for years assaulted her body so can perhaps be forgiven for trying to make up for it.
Because people's perceptions differ as to what amounts to good upbringing, people will have different views of when overindulgence occurs. Our boys were for example never drilled in the persistent use of sorry, please and thank you but they were frequently praised for their good behaviour. Friends of ours brought their two children up in a regime of robotic utterances (and quite a bit of "seen but not heard" too). One example was that they were trained to at the end of a meal always say "Thank you for a lovely meal, please may I leave the table". At least one of them developed such a speed at this that it was scarcely a sentence one could understand, much like the spiel that is electronically speeded up at the end of radio adverts ("You should check other available offers, this does not reduce your rights under law", etc., rapid fire with all gaps edited out). We almost found this disturbing but one of the children is now, for all appearances, a balanced and well adjusted adult, his sister was killed in a road accident at the age of 18.
A lot of spoiling will not be at all obvious, the extremes will be obvious. Parents all do it their way and hope for the best, some wonder where they went wrong, others die convinced they are the examples of perfect parents.