Little bar-end mirror. I ONLY trust it when I actually SEE traffic in there...don't trust a negative view...before I commit to a turn, I take a quick twist-and-look. By the way, this applies even to lane changes on a quiet double-track.
By the way...a little survival tip. [Yes, I pulled this one in my youth.] OK, it goes...quick look behind...I see "a" car back...the brain says "one" car...after it passes, I pull left into the SECOND car. [And yes, I am no longer symmetrical onaccounta' this sin and others.]