Photo of green peach aphid damage
The easiest way to scout for aphid colonies is to search perimeter vegetable plants for copious amounts of sticky, glistening honeydew coating the upper surfaces of lower leaves of plants. Honeydew may contain numerous cast (shed) skins (white to gray) and a black sooty mold (fungus) that colonizes aphid honeydew.

Photo Source: Michael Bush, WSU Extension, Yakima, WA