There's no general rule that plaques like these are "accurate" or "inaccurate". Some are undoubtedly based in historical fact, while others might present an inaccurate, incomplete, or biased version of events. You should approach plaques the same way you would approach any historical document - with a mix of curiosity and skepticism. Your question is ultimately one of historical method and gets at the larger question of how we establish the credibility of any source.

Obviously, as you attempted, you can try to corroborate the information with other sources. Are there other accounts of the event described on the plaque? What are those accounts based on? Were there eyewitnesses or contemporary documentation of the event, such as newspaper reports or military records? How would anyone know what did or did not happen at this place? Just be careful that the other accounts you find aren't based on the plaque itself (as you suspect might be the case here). Just because something gets repeated doesn't make it true!

But all that's a lot of work, and it might be more work than you want to do for a interesting plaque that you just happened to see. Let's say you're not interested in doing a deep dive research project, you just want to get a handle on whether the information is basically trustworthy. One way to do that is to see if you can figure out who put the plaque there in the first place. Was it the owner of the building? The local historical society? A public agency? Who are they, and how credible do you think they are?

When evaluating the credibility of the person or organization who put the plaque there, remember that there's always a reason why they're relaying this information. Ask yourself why they want you to know about this? Is it purely for educational purposes? Is it to honor or memorialize someone? Does the information presented further some political agenda or ideology? Would it be good for business or tourism if something interesting had happened here? Try to think about why they want you to know this information and who or what benefits from telling this story in this way.

Of course, if you can figure out who put up the plaque, you might be able to learn more about how they know what they know. Historical societies and public agencies often have the records that corroborate the story, so if you just have to know the truth, you can always try to contact a group like that and ask about the event on the plaque.

Finally, if you can't corroborate the facts, and you're not sure whether you trust the person or organization who put up the plaque, you can always just know that there is a plaque that claims that this event happened. Maybe it's true, maybe it isn't. Without more information, you'll just have to live with the ambiguity that something interesting may have happened, but that, for you at least, it remains unconfirmed.