How to distinguish if a sentence is using the historical present?
Solution 1:
The historical present is used to make a description of past events more vivid by giving those events a sense of immediacy. Since it's more a matter of style than grammar, it can be hard to come to any definitive conclusions about ambiguous cases. Newspaper headlines typically use it since the news media are in the business of being vivid and immediate, so the events of the day may technically have occurred in the past (even if it's only the past few hours), but it's more gripping to refer to them as if they are happening right now. You may come across better examples in novels or sometimes history, especially where the tense shifts suddenly from past to present in the narration of specific scenes or events, as in some battle narratives. Or when you tell a friend a story but use the present tense to create drama.
How can you tell?
- Did the event definitely occur in the past?
- Does the writer's use of the present tense make the sentence more vivid?
Your examples are all arguably in the historical present, but it's a matter of at least some personal discretion. You could also argue that the events of "today" are still technically part of the present from a newspaper's point of view. I think it's easier to identify the historical present in longer narratives that use it to achieve an obvious dramatic effect, whereas newspaper headlines are at least as much the product of attention-grabbing tactics and the constraints of space as they are of deep, grammatical introspection.
This is just my opinion! The last time I heard anyone discuss the historical present was Latin class.
Solution 2:
The latter example, "Today's Star Wars Comic Changes The Canon... Forever!" is very much a more specific version of "this changes everything."
The best way I've found to tell the difference is to look at context and consider the action that's being described. Is it clearly something that has to have already happened in the past, in order for the journalist to write about it?
For example, in "Shark dies after transport truck runs off Florida interstate" that shark is clearly already dead and the truck clearly already ran off the interstate; this is describing something that happened in the past.
In possible contrast, "Google launches Sidewalk Labs to fix cities" may be in the past or continuing into the present depending on one's perspective about how long a launch takes. While the moment the launch began is obviously in the past, it might not yet be considered fully launched, but this grammar helps a headline writer avoid small disputes about details like that.
Also, pay attention to the dates on those news stories.