For me, everything is a belief unless it satisfies the following criteria:

1. It is generally accepted as true among experts
2. There is ample evidence that is both personally convincing and leaves no room for alternate interpretations (not the same as #1, since many fields have "commonly accepted knowledge" that is generally acknowledged as most likely true but has no evidence to back it up)
3. It is specific enough that it cannot be interpreted in a way that is misleading

I find that the one that trips up most people is #3, since some people speak in technically true but overly broad statements and the listener ends up filling in the gaps with their own biases. The listener leaves feeling like their biases have been confirmed by data, not realizing that they have been misled.

In the end, according to my criteria, very little can be categorized as true knowledge. But that's fine. You can still make judgements from partial or biased data or personal beliefs. You just can't be resolute about it and say that it's true.