Photo of leaf damage from melon aphids
In addition to scouting for aphid colonies, aphids can be detected by copious amounts of glistening honeydew (sticky, sugar-rich fluid voided by aphids) coating the leaves beneath aphid colonies. Honeydew may contain numerous cast aphid skins and black sooty mold that colonizes the honeydew.

Photo Source: Michael Bush, Washington State University Extension, Yakima, WA