When you paint detail in one section too long, you loose sight of the entire drawing.
Zoom out and walk away from your desk, take a look at the drawing from afar or walk away and come back.
This way your brain will notice mistakes.
When flipping the image, this allows your mind to view the drawing differently, any mistakes will stand out like a sore thumb.
The eyes might not be level etc.
Its a great trick,
If you are drawing traditionally, hold the paper up to a window and stare at the blank side.
>> Blocking down form zoomed out.
This is a more advanced thing to do that took me several years to get used to. I paint zoomed out to shape the form and colour placement, ( which I also fix halfway through with curves)
Then once you are happy with the form, zoom in and add details, even then I only zoom in about 30-40%
Its all about preferences/ eye sight and size of canvas.