fem'd seeds used to be made by pollinating a female with a hermie... well that was the "snookie and ray ray's" way to make cheap fem'd seeds.. they often caused hermies.. but at least the male trait was gone...
now and days they use colloidal silver spray to cause true females (these are females that regardless of the stress done to them they WILL NOT hermie) to create pollen sacks...
stressing a plant to hermie and using chemicals to cause it result in very different pollen sacs forming.. at least genetically different..
a true female that has been sprayed with a chemical to force hermie will ONLY have female pollen... there will be no sign of hermie or male... when you take this pollen and pollinate another true female you get truely fem'd seeds...
NOW... occasionally you may get a hermie.. this is because it is possible for recessive traits to hide for a long time before coming out in the grow.. if this happens then you know one of the two female parents had a HINT of hermie somewhere lost in the genetics... but that is nothing to worry about.... its very uncommon and unlikely anymore.. as long as you get your seeds from reputable breeders..