Usually you'd be able to investigate certain claims by trying to find reasonable evidence for it. For instance, one might say that "red is ugly", which is not something you can back up with evidence easily. And if you can, you'll see that some think otherwise. Now, if one said "x% of the people in country Y perceive red as an unpleasant colour", then that would be closer to a fact. When personal matters have something to do with a statement, it's more likely to be an opinion. Mind you, things like "the evolution theory is true" is NOT an opinion, as it can be backed up with sources. However, saying "the evolution theory is cool" is an opinion. So, in general, you'll need to find sources for a claim and you'll likely be more able to find out whether it's an opinion or not, and whether the claim is true or not. =)
