When you're talking about a complex biological system like an animal or plant, there will often be dozens of alkaloids in your sample[0]. That makes it difficult to isolate the relevant ones and say with certainty that alkaloid x is the one responsible for the psychoactive effects, especially if the alkaloid in question is currently unknown. Then you'd need to isolate it and see if you can get the effects from the isolated alkaloid.
For a good example of historically what it took, check out the story of how Hoffman isolated psilocybin from psychedelic mushrooms:
In Hoffman's book "LSD: My Problem Child", he has a chapter (#6, entitled "The Mexican Relatives of LSD") where he talks at length about the process of isolating psilocybin and what his motivations were in participating. What you mentioned definitely played a role.
Here is a .pdf copy of the book from MAPS: [0]
If you're interested in the history of this stuff, it's well worth a read.
More generally, I think many exceptional drug chemists have been willing to bioassay the novel drugs they make. Almost inarguably the most prolific drug chemist of all time, Alexander Shulgin, bioassayed over 200 novel drugs, making him the first human being to try almost all of them[1]. What is most interesting about Hoffman in this regard is that he didn't set out to be a drug chemist: he discovered LSD was psychoactive on accident[2]. So his temperament (with regard to his willingness to bioassay novel compounds) was a fortuitous coincidence.
For a good example of historically what it took, check out the story of how Hoffman isolated psilocybin from psychedelic mushrooms:
https://en.wikipedia.org/wiki/Psilocybin#Modern
So ultimately it comes down to whether people have put in the time to isolate the various alkaloids and then determine which ones are psychoactive.
[0] https://en.wikipedia.org/wiki/Echinopsis_pachanoi#Alkaloids In San Pedro cactus for example, there have been a dozen that were extracted, isolated and identified.