I also believe they can analyze the hair follicle to determine if this was a one time thing or if they had done this to Owen and his brother many times before.
Yes, for some substances. Not sure which, but medical examiners would know. Owen has a brother who was probably treated the same way he was. Take a hair sample from the brother and analyze that... Not sure if there's enough of Owen's own hair left to check, considering the way they tried to burn the body, but if his big brother tests positive, they'll have them on child abuse charges regardless of what happened to Owen himself.
The trouble with drug testing unfortunately is that most drugs don't stay in the body very long, and the only drug that really does stay around for a long time is marijuana, which when compared to things like meth, has such low potential for toxicity and addiction that it's hardly even an issue. But ironically, it's the one drug you'd be able to detect weeks after ingestion even with a urine test. Stuff like meth metabolizes out of the body pretty quickly. (This is one of the reasons why I don't think drug testing is very useful, as companies tend to catch casual pot smokers and miss crackheads, all while believing they've caught everyone; but that's neither here nor there.)
So I checked up on hair testing for drugs a bit... interesting idea, there, by the way; I hope they think of it rather than just doing a urine test. Owen's brother would show results around now or a few days from now as his hair grows long enough for the exposed portion to exit the scalp. Methamphetamines can indeed be detected that way. And if Owen actually died of a drug overdose, then naturally his body wouldn't have had time to metabolize all of the poison.
I'm not sure what kind of a dose you'd have to ingest for it to show up in your hair, but it's safe to say that if hair testing is useful at all, it should be able to detect a dose high enough to kill a toddler. If Owen's brother got a similar dose, surely it'll show up on him too.
But then we don't actually know that the story about giving the kids drugs is even true. It's colorful and memorable, but for all we know this was just a "simple" case of neglect or abuse, or even a natural death followed by panicky meth-heads thinking they'd killed their kid... Well, okay, that last one is a bit farfetched, but it's theoretically possible.