>4chan started enforcing much, much stricter ban policies, with the unfortunate result that their site
I've personally observed this and in general 4chan's rule enforcement is in general inconstant. But I like to know a few examples of the sudden strict rule enforcement.
In regards to spotting them individually, it's the only way we can deal with the ex-4chan folk. But it is easy to spot them, posting obvious /b/ bullshit: Such as triforcing, doubles, meme sprouting, image and text spam, sage bombing.