How do I know Briiv is working if the filters still look new? Do I still have to replace them?

We generally advise that the nano-matrix filter lasts for three months, so it's not too surprising if it looks the same still! Not all pollutants are visible (in fact very few are), so you may only actually see a difference if, for example, you smoke inside as the pollutants here will stain the matrix filter a darker colour.
The moss, activated carbon, and coconut filters last around 12 months, so won't need changing for a while. It varies depending on your environment, but you may see more or less pollutants in these filters (for example an old English townhouse can be quite dusty, so the moss will gather a fair bit of dust in the time it's being used).

We'd still recommend changing your filters after 3 and 12 months for the nano matrix and coconut, activated carbon, and moss respectively to ensure you aren't keeping pollutants around for an extended period of time, and to ensure maximum efficiency of your unit.