I had a friend for about 15 years who was mildly toxic but our spouses and children were friends so I tolerated some minor things. Then one day she and I were talking and I looked at her eyes and it appeared as though the pupils were stretching a bit. Was it the lighting? My imagination?
Either way, at this point she wasn't worth putting up with anymore and thereafter I put distance between us.

The creatures can not hide their eyes!