While I don't know of any simple or convenient numerical parameter, I'd note that we do have some handy non-retrospective pieces of evidence for evolution by natural selection, such as the induced occurrence of evolutionary benchmarks such as multicellularity.

In general, there are some adaptations which are highly predictable under certain circumstances, but there may not be any sort of meaningful measure we can use for evolution of organisms over time which aren't a function of their relationship with their environment.