I do not think a test for specific bacteria would ever be feasible or perhaps even possible. A microbiologist can examine the bacteria on "x" surface and tell you what it is, but without examining every surface in the aquarium (which I trust everyone will agree is a complete impossibility to do realistically) there is no way to ascertain the number.
Second problem is that the exact bacteria species responsible for nitrification is, so far as I am aware, still not a certainty. Ammonia is converted to nitrite by bacteria of the
Nitrosonomas marina-like strain (Hovanec & DeLong, 1996; Burrell, Phalen & Hovanec, 2001) and nitrite is converted to nitrate by bacteria closely related to
Nitrospira moscoviensis and
Nitrospira marina (Hovanec, Taylor, Blakis & DeLong, 1998). Subsequent to these papers, archaea and not bacteria is being considered as the actual nitrifiers in
established aquaria. I recall reading that the bacteria likely start the nitrification but archaea actually continue it [this was a few years ago I came across this, it would take me some time probably to find it, and subsequent studies may impact it as well].
As for the "bacteria supplements" on the market, Tim Hovanec's is the only one that contains the bacteria species likely involved, according to him (no one else has proven differently, so far as I know), but of course the archaea aspect may impact this claim...?? , The other products like Seachem's Stability and Nutrafin's Cycle contain different bacteria that are not responsible for nitrification, but Hovanec examined some of these products and concluded that even though they do not contain the "right" bacteria, they did serve to speed up the establishment of the nitrifying bacteria to some extent.
There are abstracts of the three cited papers here: