I do something similar to this for many things. My go to thing is saying “CHECK” once I lock my doors to the house or close the gate, turning off a light in another room (although smart lights helped me immensely with this). A verbal or another audio cue, since most cars will beep if locked, is extremely helpful for me.
What I found helpful was saying out loud “I locked the car” while doing it.
I do something similar to this for many things. My go to thing is saying “CHECK” once I lock my doors to the house or close the gate, turning off a light in another room (although smart lights helped me immensely with this). A verbal or another audio cue, since most cars will beep if locked, is extremely helpful for me.