An odd question?

there are some spells, and even psychic abilities people can develop to drain one's energy.
in my experience, simple signs would be having a dark aura, tho not all people with dark Aura have to be psychic vampires (actually, very few of them are).
if you practice visualization and energy manipulation enough, you can use that ability as well.
for example, if you know what (grounding) is, then just think of someone who would ground on other people, simple as that.
some people do it unintentionally and do not know a thing about magick, while Others would learn it and develop this skill.

https://www.spellsofmagic.com/read_post.html?post=867198

https://www.spellsofmagic.com/coven_ritual.html?ritual=3811&coven=578

This is an essay I wrote out not too long ago about modern concepts of vampires and vampiricism. It is based chiefly on personal experience within the vampiric communities online.

Otherwise a plethore of information can also be found on www.sanguinarius.org ; it is one of the original communities, and has been around since the 90's.