Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services if the eyelid or facial appearance changes suddenly along with:
These combinations can indicate a problem involving the brain, nerves, blood vessels, or eye socket.
Do not drive yourself if your vision or alertness is affected.
Seek same-day medical care for a new eyelid change with significant eye pain, fever, marked redness or swelling, pain when moving the eye, or reduced vision.
Prompt assessment is also appropriate after a less severe eye injury if the eyelid position has changed.
Book a routine appointment if the slant is gradually becoming more noticeable, is newly one-sided, or comes with eyelid drooping.
A child should be assessed if an eyelid blocks the pupil, the eyes do not appear aligned, or there are concerns about vision or development.
A stable appearance present since birth can usually be discussed at a routine eye or primary care visit.
Every sign above is quoted from the sofidex article on Palpebral slant - eye. If you are not sure, err upward — it is always reasonable to be checked.