Study the leaves for curling and yellowing. If you see any, touch the leaves and check for any sticky residue or black soot on the leaves. These are the telltale signs of aphids.
Check the leaves for silvering or cupping. If you find this on the leaves, then they are infested with broad mites.
Check for winding tunnels on the surface of the leaves of your gerbera daisies. If you find evidence of this, the leaves are infested with leafminers.
Look for silky sacks filled with tiny insects. These are the sacks of the spider mite.
Check the tops and bottoms of the daisy leaves for tiny white flecks. Those flecks are white flies that feed on the sap of gerbera daisies.