Some are tendencies in character types. A shifter with an interesting role, a low number of deaths, and high wand knowledge is usually Lightmage, for example. ;) Really standout Heralds are usually one of a couple people, and so on.

Gearing can often be a big tell. Some players use the same food/drink/container choices regardless of character, or always re-equip in a certain order.

Choice of resting locations, levelling locations, preps, unique tactics, aggression level, RP level, favored socials, emote usage... a ton of different things can be tip-offs.

All that said, it's not an exact science. For example, I had the chance to watch Skiltore for about 10 minutes earlier in the evening, and from a few different things, I had to draw the conclusion that he was obviously me, even though that's not the case. :P