There are two very easy ones: skin color and recent ancestral origins. (If we go back far enough, we are all related.) Most people have no difficulty grouping individuals into races based on physical characteristics alone. As there is considerable mixing between the races today, the lines separating those groups become less distinct, but with modern day genetic techniques, it is possible to find a set of distinct genetic markers. And eventhough the PC left or conservatives with positive intent are trying to erase the distinction between races by calling them arbitrary or non-existent, there is an opposite trend in medical research that calls for genetically taylored treatment for different racial groups. The different groups not only carry different genetic susceptibility for certain pathology, they also carry different genetic predispositions for their response to treatment. And, indeed, there are sets of genetic markers that can identify racial groups or ethnic/geographic ancestry. And these genetic markers do correlate with skin color and overt physical features.
Skin color and/or geographic ancestry are often used to socially or culturally separate the races. This is most often done to no good end. Skin color does not correlate with intelligence or behavior the way that a polymorphism in a liver enzyme correlates with sensitivity to pharmaceuticals. And even to the extent that it might correlate, it does so as an emergent property of the whole genetic makeup of an individual (i.e. it does not depend on one or two genes) and is expressed with great variation between individuals. And this an important point: even if there is an average difference between two racial groups, the overlap of the whole group is much greater that any difference. This means that the use of physical characteristics such as skin color as an indicator of, say, the intelligence of that individual is not valid. For example, if a green race scored an average 10% lower on an intelligence metric than a purple race, a green with the median score would still be more intelligent than 40% of the purples.
There are differences between races and they have have a testable genetic basis. The boundaries between the races are blurring as the world becomes smaller and mixing occurs. My guess is that in colonial days the differences were much starker than today. Social stratification based on race is wrong in my opinion, but does not imply anything about whether or not there are races. The phenomenon of race exists, but the concept of race as a social structure is not useful.