It's certainly a confusing subject, especially now that more attention is being paid to it. We've had our share of acquaintances comment that their kids are also allergic because one time they ate something with a peanut and had an itch later that day. In our case there's immediate throat/tongue swelling so it's an obvious one.
In all seriousness, the best bet for concerned people is to see the doc and get tested. Our kid's reaction to the prick test was ~10x the normal "allergy" reaction - the doc immediately stopped the test and gave him medicine. (As mentioned elsewhere, kids do grow out of the allergy so it's important to retest!)
According to our allergist, the problem with peanut allergies in particular is that the reaction can vary dramatically for the same person. Ingesting it one time may cause nothing but hives, whereas another will result in throat constriction within minutes. Cue the news stories where people with seemingly minor allergies are suddenly in the hospital. For example, recently a girl in CA with a known allergy took a bite of a peanut butter brownie and then spit it out. Her parent, who was a doctor, waited 20 min to administer the EpiPen. She died.
(Disclaimer: not a doctor, probably didn't remember everything right, etc.)