Do you mean me? 
You know, there are often telltale signs… I’ve been reluctant to speak about them publicly (unlike game masters, botters — I mean the developers — are probably reading it and taking it into account), but for instance: they used to always target the Elven Archer into their own face if no enemy minions were present (that one was apparently ‘fixed’ at some point, so I’ll write openly about it), the way they play around certain interactions, a certain rhythm with which they always play their stuff, certain glitches in cheaper bots, never conceding even in obviously hopeless situations, etc.
As for the Influx of bots in Classic happening during the ‘Winter Veil’ event, those were even more primitive and with basic decks to boot.
Higher in legend (say, about top 300 plus-minus), you’d start meeting more actual players.
That said, I report every single one of them as a bot. Even if I’m wrong in maybe 10-20% of the cases, this is probably still more accurate then many an algorithm meant to detect them. For example, if it rains 6 days out of 7 on average, then simply saying it’s going to rain tomorrow is more efficient than million-dollar weather stations and advancded models predicting weather with 80% accuracy.
Besides, calling an actual ‘player’ with this deck a bot is a compliment, compared to what they truly are, in my opinion.
I think calling these players ‘children’ is a grave affront to actual children, who should generally be much smarter than this (not with ‘Modern Educayshun’ and all that ‘political correctness’, though; what’s more, there are even people actually proud of their illiteracy etc, who sincerely believe their ‘opinion’ about anything is as good as anyone else’s — for example, a nuclear engineer should, in their view, shut up and not believe themselves smarter or better when it comes to a discourse about nuclear power ) — and much nicer persons too. I’ve faced some clown who’d rope intentionally, emoting profusely while at it (all of it with a net-deck, by the way), for example. Admittedly, this is probably more common at lower ranks, where I landed after having quit HS back then.
I don’t know what you played, but proper chess engines (even traditional ones, without all those advances in AI like AlphaZero) became much stronger than even the best player in the world many years ago — and they’ve improved drastically since.
One more thing: some people tend to think bots are something primitive, like spamming the hero power and doing nothing else. For example:
In facts, bots were much more advanced even years ago and capable of beating even decent players (not that it says anything — in this game, a bucket of sewage with pig eyes and some decrepit hardware can win against the so-called world champion in a single game — and even in a series the difference betwen them would not be that great, skill doesn’t matter that much, unlike the aforementioned chess, for instance), see, for example, this old one:
https://www.youtube.com/watch?v=tav_ttqyv9Q
Bots in Mercenaries have already been mentioned, I believe.
That’s what all those clueless sods with cheating software like so-called ‘deck trackers’ (which in fact do much more than that, I believe, and include actual hacks) and such probably don’t realise — that they’re nothing more than lab rats to supply data to someone else’s project. In fact, some are already paying for it.
What’s the expression, ‘I see what you did there’? 
Another person discovering America, just like Columbus… through their window?
PS One more thing: all that is yet another reason to play Mercenaries, PvE in particular, if you ask me.