Consider how well evidence supports something to judge its plausibility. Given the lack of solid evidence, the plausibility of his claim is low.
Pro Tip 2/3
Not Certainty
Plausibility suggests something seems reasonable, not that it's definitely true. The detective found the plausibility of the alibi questionable, not disproven.
Pro Tip 3/3
Subjectivity Involved
Different people may judge the plausibility of something differently. Critics hotly debated the plausibility of the movie's plot.