It depends on what we call BS.... I am decent at seeing discrepancy between what someone is expressing & what emotion they are experiencing. However, I don't call this BS in itself, because I think choosing to go with a principled response over an emotion is part of being a mature adult. I also need to get a sense of someone over time, to see what is consistent & normal for them, before jumping to any conclusions.
In other ways, I can be quite naive. I've recently gotten more sensitive to being emotionally manipulated, because after having been a victim of it I realized I give extensive benefit of the doubt to the point where I gloss over such discrepancies as noted above. If I had given them more weight, I may have seen enough inconsistency between expressed feeling & feeling in demeanor & action that I would've judged someone as NOT truly concerned with my benefit. I suppose that is my main aim in any consideration of authenticity - is this person's feeling towards me genuine?
I don't associate this with Fi, because when I was younger - arguably before Ne took on a larger part of my thinking - I was very absorbed in myself so that I did not read people at all. I didn't & still don't really use Fi for direct judgment to outward things - it's more like I use Ne to see the various ways it relates to my Fi concepts of the human condition, and blah blah blah. Anyhow, when jolted into reality, I tended to be hyper-sensitive & take things negatively which were not, but reading authenticity was not even something which dawned on me. This sometimes led to me being acutely disappointed when I thought someone liked me or was being kind, and it was not genuine.
I notice people who feel they read others well are actually P-dom (ExxP & IxxJ, especially the Feeling varieties). Things simply are apparent to them & they trust this very quickly. For me, I need to go into analysis of my perceptions to "make sense" of things & apply the appropriate weight to it all, and I hesitate to do this too quickly, for fear of jumping to conclusions. So this suspension of final judgment prevents me from decrying someone as full of BS. Even if I am becoming aware of it, unless it appears as damaging & in urgent need of review, then I don't really even label it to myself.