Well, when you watch them long enough, you know.
Seriously, you can tell that it is any species is a hybrid when you see the following things:
1. You see them breeding together and they have offspring.
2. The offspring has characteristics of both species and you know that both species interbreed.
Hybrids, including domestic-mallard hybrids usually have traits from both species. Gadwalls, for example, have white speculums, a higher forehead and narrower, darker bill than mallards. Hybrids will often have white speculums like the gadwall, but the same feather patterns as a mallard. Or, they will have a gadwall shaped head and bill, but the bill might be orange or greenish. Male gadwall-mallard hybrids will have the "puffy-looking" head that male gadwalls have (male gadwalls' heads don't turn green), but will be green or mostly green like a mallard.
As for the killdeer, you can tell them apart just like you do people-- they all look different! They all have different markings, body shapes, voices, and personality. Granted, some of them look so similar that I have to tell apart by comparing photos, but they each have different characteristics. And, believe it or not, there are even a few characteristics and marks that they keep from hatchling to adulthood.
OK, once you know who's who, you can follow their families. You can see them chose their mates, have chicks, watch their chicks grow up and the cycle begins again. I can go into more details about how to determine gender, differences between juvenile and adults, etc.
I hope that answers your questions.
