here in the uk we are pretty tightly regulated with any one who wants to train other operators having to be certified to instructor level. thats normally a two week course for cb and reach truck. the way insurance companies try to find somebody to put the blame on, any one who is even thinking about training other operators would be best advised to do as alabama says, follow the best practice you can, keep records, do your refresher training, have public liability insurance and just hope that the majority of the people you train will remember the training. some certainly wont.