The FBI UC guys were easy to spot: dry cleaned jeans, and they smelled better. Some, if not all of that probably applied here too.
Most LE agencies, investigative journalist, PI’s, etc have become much better, more sophisticated since the 60s, in regards to blending in, playing/dressing the part, gaining intelligence, not drawing attention to themselves, using better covers, pre-planed plausible denials, others use very creative door opening pretexts, are able to extract info, etc.