In the Netherlands its possible to test you illegal drugs for substance and potency; and there the testing results of other chemicals on blotters are pretty rare; it does exist here too but its just very uncommon; personally I had all my blotters tested there and I've only had real LSD, most people I know do.
The dosage testing on LSD isn't too certain by the way, we don't know how much of the unstable chemical they broke during the testing or storing it to be tested; it gives a decent estimate dose though; I mean if your blotter had been tested at 50 ug or less, you know you'll have to take 2 or 3 for a really good trip, and if it tets at 200 ug you know its a bad idea to take 2 or 3 hits, so it still helps
Either way, I díd have a 16 hour trip on real acid. I had it tested that time too, actually becáuse I was doubtful because of the duration, but it turned out to be just a high dose of LSD, so its not that if your trip is long its always another chemical. I did also hear that DOx-es have a remarkably long come-up, my 16 hour trip started in 10-20 minutes, as high doses of LSD always show their early signs really fast.
I'm not sure how you can find out without the testing, but I've had long lasting and bitter hits that were absolutely really 100% LSD; also tasteless ones by the way, but bitter doesn't mean other chemical, it might mean it, but definatly not for certain.
I think you have to find out by the effects themselves, and if you have no access to purity/substance testing of drugs, I'd say keep the first dose of a new blotter lów, because LSD has a huge therapeutic range (meaning you can't overdose easily), while DOx substances have a much shorter one, so take 1 hit of a new blotter and not more, maybe even just half to make sure of what you're getting, if you can't test. And if you cán test but are impatient and don't want to wait for results, the same thing does.