Here are two somewhat similar-looking warblers:
There are a few things we can look at here to distinguish the two, like color, eyering, etc. but what I'd like to point out relates to contrast. Contrast is just the difference between light and dark tones. For example, a bird with all black and white markings (like this Black-and-white Warbler) is very high contrast, while a bird with just intermediate tones (like this Orange-crowned) would be considered low contrast.
So for our original birds, let's look at the throat. One bird has an isolated pale area in the throat that contrasts strongly with the head and body, but has a lower-contrast eyering. The other has a throat that blends more seamlessly into the body, and hence is not isolated, but has a higher-contrast eyering. In this case contrast is about tone, not color, so don't be fooled by the brighter yellows of the lower bird - the throat is still less contrasty. To that end, we can see the throat and eyering contrast even more clearly when we remove color and just look at the birds in black and white:
The bird with the high-contrast throat and duller eyering is Common Yellowthroat. The bird with the lower-contrast throat and bright eyering is Nashville Warbler. These marks can be observed very quickly, and with these small, flitty birds that can be important.