When ever I'm a passenger in a car, as well as checking myself (as well as I can, sometimes it is difficult to look over your shoulder on the window side if you're still strapped in) I ask the driver if it is clear to open the door. They can check the mirrors and check behind easier than a passenger.
I've not had the misfortune of being doored but often do filter in both the right and left, depending on where I'm going at the junction. If left lane is left and straight on and I want to be left I'll filter in the left, for instance.
+Another for covering the brakes and being aware of peds crossing, vehicles turning right, (across your line of travel, to your left) and the passenger taking the opportunity to get out whilst vehicle is stationary. There is always going to be someone who'll do the unexpected.