Unless they write it clearly like your examples there's no way to know without asking.

Even if they did it personally that's still selection bias and doesn't necessarily change how valid their statement is.

If you just want to know, without further investigation, that's going to be very hard. People say a lot of things, and often aren't clear themselves if it's something they actually know, or just something they've heard. All that's happened is that something interesting / helpful has popped into their mind and they've shared it.

If you are willing to discuss it, but don't want to be rude by asking "do you actually know anthing about this?" you can just ask follow up questions, asking for more info / details. That way people who really know can answer, and people who don't will probably just not answer or say that they just read it somewhere (often they're not trying to deceive, just sharing something interesting they heard about).

But as others have said, just be sceptical of stuff you read, especially on the Internet. Lots of people have first had experience of something and still have unhelpful or strange takes on things. People massively over estimate how representative their experiences are, and if you get two experts in a room they'll pretty soon be disagreeing about something they both know throughly.

You could ask the followup question "So you personally tasted an orange and it was sour?"

So, thinking they are automatically lying is even worse than thinking they must be telling the truth. The position you need to hold is between the two.

The reason to hold it consistently is to take advantage of habit building and using how your brain works to your own advantage. You can try to calculate an independent "likelihood" for every claim if you want, but you'll frequently be wrong, just because you can't take everything into account. And it's a massive waste of energy.

As to why, it varies. Humans are very different from each other, so the reasons will be many and varied. But the important thing to remember is just how easy the lie is, and how there's really no consequences if someone does.

Nice try. That other person told me nobody online has ever eaten dragon fruit.

Yes, and sometimes you have to throw in a real curve ball!

By the way, as head of quality at a saltworks in Europe, I should point out that there are as many shapes and sizes to processed salt as there are subtleties to their trace mineral concentrations. So "a grain of salt" isn't a well defined quantity.

I can't recall for certain but I'm pretty sure at least one of the far northern american languages has it. Like something from Eskaleut or Athabaskan. A quick scroll through their wikipedia pages didn't help, and I can't remember the formal name for this feature. If anyone else can chime in, that would be appreciated!