Video evidence. Then Merseyside Police get a lip reader in to confirm what has been chanted. .
Another thing Merseyside Police do. They take the person outside Anfield and charge them there instead of inside the Anfield. That way they can add/ask for an exclusion zone around the away fans home grounds.
Merseyside Police are absolute cunts. They will use every trick in the Police book to charge opposing fans with additional charges.
Yet they turn a blind eye to what Liverpool fans do and chant. As we all know.
It's not just Merseyside Police. LFC have plain clothes spotters in the away end, and stewards watching away fans chanting. I've no doubt the home stewards that accompany away fans to Anfield are also looking for people chanting.
Anfield is the most watched away end in the PL. In fact, it's become an obsession and part and parcel of the match day routine for Liverpool staff, stewards, and Merseyside Police. It's paranoia.