@lapingvino If they're offering their advice in American English, that they're in the US seems as good a default assumption as any. I think it might make sense to clarify if they're speaking some language that suggests another origin.
On the other hand... free advice is worth what you paid for it, right? So you should exercise due diligence in verifying it *anyway*, even if it comes from someone you know to be your countryman. If you follow advice from random strangers on the Internet, "it assumes I'm American" is probably not the failure mode about which I'd worry most.
@lapingvino I've seen such situations too - but I still think the marker is useful despite not being infallible.
The most reasonable norm would seem to be for the recipient of advice to ask the giver as to his whereabouts, if he suspects it might matter. That seems nicer than expecting someone to disclose his location as a matter of course, especially when he's doing something to help others.
It's not just Americans who mistakenly assume their experiences are universal - my favourite example is actually Australians trying to get their minds around the northern hemisphere school year. That there isn't a perfect one-to-one correspondence between calendar year and grade level is hard to imagine without experiencing it for oneself.
@khird I have seen situations where American English is spoken and literally no one is American. Language is not a useful marker here, because MANY countries default to English online nowadays.