Difficult to measure... There are lab tests you can send out but I personally have never used one. I know it's almost like talking "vapor ware", but it's true. This is why for years folks only used live rock in their tanks. That rock was loaded with bio diversity. Unfortunately it's usually loaded with bad hitchhikers too. That's why I start systems with dry rock that I "cook" in bacteria for several months before starting a tank. Every time I have started a new system or did a transfer I add various types of bac / micro life to the system over the first 6 months. Example: My present system is an AF system. I dose BioS, Pro BioS, -NP Pro (carbon source), AF Life Source, MB7, Microbacter CLEAN, Pods and Phyto. This gives the system a chance for the positive organisms to take hold and stave off the bad stuff. You know everything is working when the tank becomes stable, meaning no outbreaks of the bad stuff ever (Dinos, diatoms, nuisance algae's, cyno). The tank starts to remain "clean" for much longer periods of time and you don't have a build up on your sand bed. Hope this helps, but the whole biome cycle goes way beyond the initial nitrogen cycle of the tank.