I normally just tell people. Disdaining to conceal our aims and so forth. For those times when outright saying I'm a socialist would derail things into an unproductive debate about how "communism has never worked" or whatever, I usually default to saying I'm an anti-capitalist or something like that. Even then, I try to sneak in surplus value and class conflict and similar language. If I'm fairly certain I'm talking to another leftist, I let it slip as quickly as possible so we don't spend however many minutes using coded language and sniffing each other out.