The actual moment when a "daughter" species can (or will) no longer cross-breed with the population from which it sprang (the definition of speciation) is almost impossible to pinpoint, let alone to witness.
One early-warning system: inserting rodents with depressed immune systems (known as "nude sentinels, " because they happen to be a hairless breed) into the general population of rodents--canaries in the coal mine, if you will.
If people in this population meet and breed at random, it turns out that you only need to go back an average of 20 generations before you find an individual who is a common ancestor of everyone in the population.