I really don't know, but think that it is rare enough that it is an individual tolerance sort of thing. What I mean is that perhaps there are a few that tend to absorb more than others, otherwise, you would find a lot of documentation in the literature. This is a drug that is used a lot as an OTC drug. Granted, most people do not take it chronically.
But, if you do take it chronically, then it is just smart to be alert for any changes in your cognitive ability, personality (irritability, mood, affect, etc.), that sort of thing. If you have any concerns, THEN testing would be appropriate. Much better than going through all the baloney this woman in the link had, with invasive neurological tests before testing for bismuth levels.
Or maybe it might make sense to get a baseline now to see how you are handling it, since you already have been using it chronically. That way, you might get an idea of whether you are fine to continue as you are, or maybe you should reduce your daily dosage, or even go with every other day dosing.
Jan