As hobbyist we generally do not have reliable means to quantify the effectiveness other than using our eyes (waste buildup, algae, unhappy plants and fish etc.). There are ways to get at least a sense of the situation however; One approach is to measure Oxidation-reduction potential (ORP) - I did myself for a while and occasionally still do a measurement at the substrate level. ORP especially if measured along side other parameters such as Dissolved Oxygen can help determine a biological systems ability to break down waste. But it's arguably very hard to interpret the results from an ORP measurement by itself.
For a moderately to densely planted aquarium the active bio filtration from plants and substrate (unless you are deploying a massive external filtration system) far out weights the importance of external bio filtration - but of course everything counts - many prefer the
belt and suspenders approach.
It depends... more is generally better
Cheers,
Michael