If this just happened I would have a certified arborist come out to look at your trees. It will be very hard if the trees died a while ago and there is no evidence to look at. If it is worth it to you, contact a certified arborist that does consulting work.
Generally, the type of pesticides that usually do damage are herbicides(weed killers). It would really take a large amount of insecticide being oversprayed or drifting to cause death of your trees.