In the DA Universe, there are fade spirits. The bad ones are called demons, the good ones are just called spirits. If you're calling it a demon, that's because it's bad. If it wasn't bad, you wouldn't be calling it a demon, you'd just call it a helpful spirit. Obviously a fair number of the demons are named after the deadly sins, which makes it even clearer that they're bad, and why they're bad. Not all the spirits are bad, some are good, you wouldn't mistake the good ones for a demon (but you could mistake a demon for a helpful spirit, if they're trying to trick you like in the Mage Origin of DAO) . I don't know whether there are more good or bad spirits, but clearly there's plenty of both. Solas has a bit of a hard time telling the good from the bad, he tends to think of all of them as good, whether they're good or not, but of course, without giving any spoilers, I'll just say that he clearly has a hard time telling good from bad in more than just that one area of his life
. So yes, bottom line, demons are bad, helpful spirits are good, and usually it's not all that hard to tell the difference, at least for most people
.