Commercial vehicles are usually subject to permit and weight checks when entering each US state or Canadian province. However, California differs from the norm. On most of the main highways into California there are fruit inspection stations where all vehicles, both commercial and private, must stop. It is not allowed to bring any fruit into California, due to the risk of infection. Any fruit in the vehicle will be confiscated, without any compensation.
Inspections are usually very brief and full vehicle searches unusual, but certainly possible.
Therefore be sure to only purchase fruit that you can consume prior to entering California by road. In recent years the fruit inspection stations have also offered travellers a free California state highway map. There are no inspections on leaving California by road. I suspect that similar regulations apply when entering California by plane.