As someone who has held hiring responsibility: there's a difference between asking a prospective employer for the information needed to make an informed career decision and challenging your prospective employer to convince you of its prospects to make you wealthy.

There are a number of perfectly reasonable questions prospective startup employees can and should ask that will give them the information needed to run their own exit scenarios. For instance, "What percentage of the fully-diluted outstanding shares does my option grant represent?" is always a good question to ask.

A prospective employee who demands that the company guesstimate how much he or she might make under random acquisition scenarios occurring at some unknown date in the future will stand out as being inexperienced at best and unreasonable at worst. Furthermore, no hiring manager can ethically meet such a demand.

In this case, the OP is already employed by the company he's negotiating with and it sounds like he's the one asking for equity. To request equity that hasn't been offered and then demand that the company prove it has value is a particularly awkward negotiating stance. Ostensibly he's asking for equity in the first place because he thinks it might have some value.