Farm workers pick lettuce in the early morning hours near Huron, California.Photo: Jeremy RueA young female farm worker picking fruit in Washington’s Yakima Valley came to see Dr. Matthew Keifer after pesticides being sprayed in an adjacent orchard wafted onto her. She arrived with red, swollen eyes and itchy, irritated skin -- classic symptoms of exposure to Paraquat, a common weedkiller that can cause kidney, heart, and liver problems. Keifer suspected the Paraquat had made her sick, but proving those suspicions was impossible: For many pesticides, no tests exist that would show, definitively, whether or not a person been has …