Having done FBS batch testing for some 30 years here are some of the things I have done in the past.
We have exclusively used New Zealand (NZ) sourced FBS/FCS for all our work. It consistently out performs all other serums from throughout the world. It is however the most expensive and hard to get serums. The methods used over the 30 years have changed due to our work and the exquisitely sensitive assays that have become available over that time, both commercial and in house. However, these are the methods our lab have used recently (over the past 10 years):-
Oxygen consumption of 3 different cell types using an oxygen electrode
ATP production by the same 3 cell types
Intracellular Cytochrome C concentration (in house method)
Induction of inducible Nitric oxide synthase in 2 cell types
Concentration of cGMP in smooth muscle cells
In all tests one batch of NZ serum always out-performs the others consistently. If you use very insensitive methods to measure cell activity (cell viability and growth) then you see little difference from any of the commercially available serums.
The Professor in charge of my lab wanted consistency not only over experiments performed at the present but over decades. Many researchers are normally ignorant of the differences between serums and is a major reason why it is sometimes impossible to replicate experiments from one to lab to another
I do hope this helps a little