[bigbugbag]

This used to be true but is not anymore, look at the following warning erowid has added on its LSD vault and LSD FAQ.

NOTE: Some blotter and liquid LSD being sold in 2013 in the Americas and Europe actually contained NBOMe compounds such as 25I-NBOMe. These chemicals are active under one milligram, but can cause strong effects even on a single 1/4" (6mm) square. The blotter or liquid with NBOMe compounds is usually identifiably bitter, where LSD-containing liquid or blotter has a very mild metallic flavor or no flavor at all.

ADULTERANTS:

[Erowid Note 2014: Note that the following section is no longer accurate as of 2010. Please see Spotlight on NBOMes: Potent Psychedelic Issues for a little discussion of other substances now commonly sold on blotter. The Erowid Crew now estimates that there are over a dozen different chemicals sold on blotter the same size and styles as "acid-style blotters" of the past. These include NBOMes, NBOHs, etizolam, phenazepam, AL-LAD, LSZ, Bromo-Dragonfly, DOM, DOC, DOI, and others.]

[pantalaimon]

But that's the very thing I have written. Those blotter with NBOMe will have no LSD on them and I haven't heard of any that has NBOMe in addition to the LSD either.

So if you buy from a trusted source (a product with lots of reviews on a dark net market) you can be rather sure that you are getting what you ordered.

There is also the option to use a test kit.

https://www.erowid.org/chemicals/lsd/lsd_testing3.shtml