Obviously in a controlled, medical environment the problem is void.
NB: I am NOT advocating drug use/abuse of any kind. This is a legitimate question I've been asking myself for quite some time.
Another way is to pick or grow psychedelic mushrooms. Before picking wild psychedelic mushrooms, I strongly suggest you learn a lot about how to identify them (ideally not just from books, though good books are important, but from experienced mushroom hunters). Picking wild mushrooms can be very dangerous. As the saying goes, there are old mushroom hunters and there are bold mushroom hunters, but there are no old, bold mushroom hunters.
Yet another way is to go to a country where these substances are legal, decriminalized, or tolerated. Until recently, the Netherlands effectively allowed the sale of psychedelic mushrooms. Erowid's Law Vault may help you find other countries with less idiotic drug laws than the US.[1]
Finally, you can submit a sample of your substance to, or get testing kits from companies such as: [2][3]
[1] - https://erowid.org/psychoactives/law/law.shtml
Now, DOB is very bitter, and LSD has no taste. If you put a piece of blotter in your mouth, and it tastes bitter, just spit it out -- that's probably DOB. Otherwise, it's LSD, and it's pure.
I didn't know that LSD wasn't ever supposed to taste. Figured the bitterness was just from the ink on the blotter. Took three tabs, felt it a little (comparable to 100ug), figured it was just weak acid and so redosed five more tabs about two hours in. Even on heroic acid trips the longest I'd been away was eight hours. On DOB I stayed out for a solid 24.
However, this particular DOB was distributed in pill form, so people believed that it was ecstasy, so I saw a lot of bad things happen to people from it.
Its a really interesting psychedelic (in my opinion) but definitely needs to be handled with care. You need at least 48 hours after taking any appreciable dose to return to "normality" so if you are taking it, make sure that you have at least two days free.
The point about bitterness is absolutely correct though, and certainly any purported LSD blotter that tastes bitter should be suspect.
Interestingly, at least one non-psychedelic has been found distributed on blotter paper: alprazolam (See http://www.justice.gov/dea/pr/micrograms/2008/mg0508.pdf), for which I'd tentatively expect the bitterness rule also to work.
Edit: This PDF: http://www.bvda.com/EN/download/np_instructions_2002.pdf says the test indicates the presence of ergot alkaloids, not tryptamines.
It is however true that there are substances which both fit on a blotter, and are indole psychedelics - e.g. 5-MeO-AMT, or other (other than LSD, that is) ergolines, such as LSB, ALD-52 (though these are extremely rare and (likely) share the safety profile of LSD). The Ehrlich test is not capable of distinguishing between different indoles, so you may get a case of mistaken identity in this scenario.
Both the blotter paper and the inks on it could have a taste. Also, the sensation of taste (or lack thereof) could be influenced psychosomatically, by conscious or subconscious expectations.
Anyway, by the time you taste it, it's too late.
It's certainly preferable to identify a substance before tasting it, but it's not too late - spitting the paper out and rinsing the mouth out is likely at least to reduce the absorbed dose.
A perhaps more pressing problem is the risk of anxiety due to an innocently-bitter piece of blotter - I imagine a borderline-bad trip could be made much worse by a persistent fear that it would last >24h.
Of course, real caution would mean not eating anything cooked up by criminals in an illicit lab...
The times when you could be reasonably sure you are getting LSD if you are getting the drug on blotter, are long gone. There are many drugs that can be/are being distributed on blotter paper. Some of these have a very steep dose-response curve, and can be very dangerous in relatively minuscule amounts.
While it's perfectly safe (if a bit reckless) to consume, say, upwards of three hits of LSD on blotter, doing the same with blotters containing 25I-NBOMe or Bromo-Dragonfly could land you in hospital or even the mortuary.
Taking psychedelic drugs originating on the black market has, unfortunately, become an intricate game, and you really need to learn the rules before you can play it at least somewhat safely. There is no easy answer, other than doing your research, preferably lots of it.