Deriving Fixed/Variable Costs with Limited Disclosure

I'm analyzing a private company with very limited disclosure. While its fixed/variable structure and incremental/decremental margin are not all that material to my analysis relative to considerations regarding the industry itself (secular trough), I wanted to know how you go about modeling this and whether or not it is worth it at all to do so and guess, compared to stating outright to my PM that the information provided is inadequate to assess this at all. Thanks in advance.