Aphids are easy to find, just look for the ants. Ants are opportunistic little insects that love a sweet and easy meal that the aphids provide. When the aphids begin to feed on the plants they secret a sugary substance called honey dew. The ants love the stuff and tail the aphids to take advantage of their leftovers. Ants to the same with scale and other sucking insects that create honeydew residue. The incredibly strange yet interesting thing is that some ants will actually store the aphid eggs over the winter to save them for spring. That way the ants can harvest the honeydew from the aphids in the future. Ants will even move the aphids from stem to stem to harvest more from the juicy plants!
Pesticides will work against aphids but aren't necessary. A strong blast of water from a hose will dislodge the aphids from their meal. Insecticidal soap will do an excellent job of decimating the aphid population. If the plants are mature and well established the aphids will probably do little to hurt them. Still, I'd rather knock them out when I see them! I've had aphids appear on the hostas, asclepias (Butterfly weed), spirea, viburnum, and several other kinds of plants. It's one of the unwritten laws of gardening: if you plant it, aphids will come!