This question is as simple as the title reads. In general, if I know the FG of any given beer, can that be a good indicator to me of how sweet that beer will taste? So the higher the FG, the sweeter the taste.
My guess ... 80% of the time that will be true, but not always. I'm thinking there are cases where a beer might contain a lot of unfermented dextrins contributing to the Final Gravity, which do not provide a sweet taste.