scw and jmyoung - that certainly is what I do and will assume (that the person has no evidence if not mentioned), because I think it's true for virtually all cases (and I'm not going to waste my time on the very remote chance of a tiny nugget in all that mud, unless it's a fact I consider critically important - and when I have followed up on such points, I never got any result that was helpful).
Recently I happened to look at someone's tree and there was a note that the date of death of a particular individual (who interests me as well) is unknown. I did some checking in a database where a county death register had been transcribed, and there he was - so I sent the tree owner a link and the dates. A few days later I looked back at that profile, which now had the dates but no mention of where the information could be found and verified.
So yes, I know that sometimes there is some actual evidence behind apparently unsupported assertions in trees, but most of the time there is not - and it's just not worth it (to me) to try to contact every tree owner on the chance that actual evidence might exist, because I know (feel) I would end up chasing ephemera probably 99% of the time.