You determine baseline by testing oil when new. All results of in-service oil then compare to the baseline and on that basis evaluate the serviceability of this oil. The list of tests depends on type of oil, its use, and environmental and operational conditions at which your equipment run.
Lubricant analysis depends on both physical and wear metal / contaminants analysis.
Decision on physical parameters is more on spot values where as wear metal / contaminants is on trend. For eg. if BN goes below more than 60% of the original value then action has to be taken immediately. Whereas if Fe or Al content has increased in a report, we cannot infer anything. We will have to wait for the next report and then decide.