According to the OSHA standard (29CFR1910.178):

The employer must keep a record that shows that each forklift operator has been trained. The record must include the name and signatures of the operator, date of training, date of evaluation and the name of the person(s) giving the training and evaluation.  Refresher training must be given if the operator has been involved in an accident, a near miss or unsafe operations. Also, if an operator is assigned to a new type of forklift or if workplace conditions change and an evaluation shows they are not operating it safely, then refresher training is required.