This is what is on my easel at the moment. I’ve just changed the ear location. I often get quite far into a composition before I notice serious errors and I have three seriously good critiquers that help immensely. With a landscape it doesn’t matter, I usually just incorporate any errors into the composition and it doesn’t make much difference. With a portrait these errors can make or break the likeness.
I continuously compare the image to photos I have, to find inconsistencies. On occasion if I’m having real problems I will print a photo at the appropriate size and mark appropriate points with a pin. Typically these are the left and right points of the eyes and the location of the mouth and nose. It takes very few points to get the layout. This is rare but sometimes the relative points just don’t come together and I’ve never been able to use a Camera Obscura or a Camera Lucida effectively. Artists have used a similar techniques in the past but I do think of this as a bit of a cheat. It’s similar to transferring a design onto wood before carving. I haven’t had to use this method for this portrait. It can help to turn the painting upside-down. My daughter’s advice is to turn the painting in 90 degree increments until the problems start to appear.
I also have a small mirror in my studio that will make some problems stand out. The mirror is at the back of my studio so I just have to step back and look at the painting in the mirror. It’s amazing how often a horrible mistake, that I’ve become immune to and no longer see, will stand out using this trick.
I use every trick that I can because sometimes I need it. I’m quite sure the great masters used them too but I don’t make the mistake of thinking that equates me in any way with the great masters.