One way to know is to look for clear indications within the comic panels or dialogue. Maybe there's a scene showing their death or other characters talking about it. Also, if they don't feature in subsequent story arcs, it could be a sign they're no longer alive.
Well, sometimes it's obvious when a character's death is a major plot point. But other times, you might have to pay attention to subtle hints like a memorial or references to them in the past tense. Another clue could be if their powers or abilities are passed on to someone else, suggesting they're gone.