During World War II, the telegraph interception guys would figure out which enemy units were where, even without having broken the codes, because each telegraph operators each had their own "fist," or distinct patterns in how they punched in the Morse code, and people listening to the signals day in and day out could learn to distinguish them even when dealing entirely in encrypted text.

In modern times, attribution of hacker groups include other indicators include what time zones certain people seem to be active in, what their targets are (and aren't), hints about installed language support or keyboard layouts or preferred punctuation or localized representations of numbers. For example, you can tell here on Lemmy when someone uses different types of quotation marks a decent indication of what country that person might be from, even in a totally English language thread.

For more on that, go listen to Darknet Diaries, episodes where they talk about major hacks are great