Blood Suckers from Hell

Rumi speaking to a crowd muses:

Watch out for those blood suckers from hell--
cause they're everywhere.

And the crowd wisely retorts:
That sounds serious--What do they look like, any hints;
Are they usually disguised?

Rumi again: Yes, usually, they are awfully tricky!

"How then to detect them?"

I have noticed
their eyes will narrow and their faces begin to squint
like prunes

if they hear good