In addition to relying on experience to tell me when I'm clear, I try to give the daydreaming, cell-phone talking, unaware drivers as much warning as possible.
I have added high intensity LED clearance lights down low on each side, mounted on the wheel skirts and a "pickup truck light strip" sold by JC Whitney across the rear below the big window. All are wired into the L&R turn signals at the regular tail lights. I try to put on a turn signal as early as I can, and when I do I have lights flashing in unison all over the back half of the trailer.