It's all relative. You have to first know if everything is built straight, and then, as you said, whether any offset was built into the firewall.

You also need to know the incidence angles of the wing and stab, so you know that "2 degrees down" may mean "two degrees down, from a 90 degree firewall, or "2 degrees down from the aircraft waterline", or "2 degrees down from either the wing or stab incidence setting".

Most standard Pattern-style "Trim Charts" can help you figure this out visually, by actually flying the model, which is the most reliable "proof" of what's needed.

I think once you become more aware of trim changes in flight, you can judge what the plane needs by just looking at it carefully in flight. (and jotting down a few "behavior" notes while you're at the field doesn't hurt)

I always start with the angles suggested by the designer, then adjust if needed.