Here is a simple open-source site (FD: which I wrote) which handles this for you: https://friendlyoptions.org/
Give the site to your prospective employer, ask them to fill it out with the details of your stock option grant and then share the link with you (all the information is stored in the URL so no data leaks to the site --- or just run it locally if you are paranoid). The site asks for the minimum amount of information possible for you to start valuing your stock options so if they can't fill it out, you essentially have no way to value your options.
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With that advertisement out of the way, I disagree with this article a little bit. I think questions about the past/present are all completely fair game and should be asked and answered:
* How much money have you raised and at what terms?
* What is the current stock option plan and what are the details of my stock option grant?
* Have their been any secondary sale transactions in the past? Who participated?
But once you get into the future, prepare to be sold a very optimistic vision of the future that may or may not coincide with reality. And this isn't necessarily anybody doing anything wrong, generally the people who run these types of companies are very optimistic people who believe that what they are saying is true. However, this doesn't mean it is likely to happen. I'm not saying you shouldn't ask, get all the information you can, but a company will never tell you a pessimistic view of the future (and nor should they). They may say they'd never take debt and are shooting for an IPO in 2 years, but I guarantee you if something changes in a year those plans might all go out of the window.