If you want to know if something is plausible, you think about an event in your story and compare it to real life. If it's not something you see everyday, then it's not plausible.
Uh, Blue, you're writing a story with magic in it. By your own definition, your story would be implausible. By that definition, stories about magic, vampires, werewolves, genies, dragons, etc. wouldn't exist. The story has to be plausible within its own boundaries.
Thrillers often take implausible stories and turn them into rocking action stories. In one of Clive Cussler's books (Sahara), the story has a subplot that the death of Lincoln didn't happen at Ford's Theater and was covered up. Instead, in the book, he was on a Confederate ironclad that got stuck in the Sahara desert. The author lays in all the groundwork so that this storyline works.
Is there a way I can tell if something is truly implausible, or will I need to ask for third-party opinions?
I think just make sure your character's motivations for doing things in the story are solid, and keep them realistic in the boundaries of the story. Remember there are people out there who don't find the characters in a romance realistic, but those aren't the audience for a romance.