An incel is a loser who can’t get laid, mainly because they are convinced they’re entitled to sex even though they’re massively unattractive and don’t even have a decent personality to make up for it. They are also known as “nice guys”.
One way to spot an incel is that they get extremely angry when you turn them down for sex (or even just ban them from a discussion forum).
Of course, the easiest method is to spot them being run-of-the-mill misogynist pigs.
[See this entire post about pedopologists for examples: Bulgingsnake vs. Wishin-he-wasn’t-a-pedopologist Deathmatch!]
Incels on the internet are abundant. Now that you’re armed with the information you need, you’ll be able to spot them immediately!