For me... the first "uh-oh" moment came at about age 3 when we got the report from his teacher that he had "ungodly concentration".
Most toddlers don't. They have a 15 minute max attention span. ADHD kids, however, all have something called "hyperfocus"... which translates to "ungodly concentration" in *anything* they find INTERESTING (which is both the flipside to hypofocus/distractibility and a big part of why most adhd kids are gifted in some way).
Over the next several years there were a number of "tells". Including, but not limited to emotional volatility, sensory "schtuff", ignoring social "cues" (aka class clown/ standing up to bullies/ fixating on single aspects of a situation/ etc), only listening to part of what's said, leaping off on "exciting" tangents, giftedness, oh gosh... hundreds of little things here and there... both on the positive side and negative side of the disorder. All of which was in addition to his always being an extremely active / intense child.
When did I actually *know*? The first time I decided to try an experiment and gave his a mocha (instead of hot chocolate) and we spent the next several hours flying through work he hates/ considers "boring".
Since I'm adhd-c (OTC medicated... meaning caffeine and nicotine), I had a LOT of "uh-oh" and "huh" moments. But I didn't want to tag him (my mum had years prior, while I was still in my "probable" phase)... because I didn't know how much was just learned behaviors from ME in conjunction with his own personality. ((<Laughing>... waiting and observing for several years myself ... I would never say you're "jumping to conclusions")). ALSO because I'm adhd-c I started teaching kiddo coping mechanisms since he was a toddler, plus went out of my way to make sure that as much of his life as I could was suited to him (certain things bring out all the negatives of adhd, while certain other things bring out all the positives). But really, the opposite caffeine reaction, just nailed it.
BIG second to CAWriterMom's post. *I* don't use meds purely because I haven't found one I like that ALSO lets me keep the positive aspects of the disorder. Many people DO find their "personal magic wand". But I've been on several that I really and truly adored except for ________ which just made it a no-go for me (ex: on my favorite to date, I can't hold 5-6 different thoughts in my mind at the same time, which means I can't write... on another I couldn't multitask... etc), and for my son we haven't used meds YET (we may very well at some point), because currently a) adhd isn't a problem at our house (because we're a very adhd household)... just a series of challenges to work around... and b) Since I've been dx'd for over 20 years I personally prefer to teach the coping mechanisms FIRST and c) we have the luxury to be able to since I pulled kiddo out of school in the 1st grade. For some reason, soooooooo many people make moral judgements on adhd meds. Which I just don't understand. I mean, no one makes moral judgements on Insulin or antiseizure meds.
www.additudemag.com is a great resource. And my personal favorite adhd book is by Kate Kelly & Peggy Ramundo... "You mean I'm not lazy, stupid, or crazy?!?"