Mealybugs secrete sticky honeydew that makes the leaves look shiny. Leaf distortion, spotting and growth deformities are all symptoms. Scales are hardly recognizable as insects at all. Some resemble tiny oyster or turtle shells. They come off easily by scraping them with your fingernail.
Scales can also leave a honeydew residue.
Aphids are most common on tips, buds and under leaves of plants. They can be winged or unwinged. They secrete honeydew. Plants infested with aphids will have curled and distorted leaves. A whitefly-infested plant will look wilted and faded.