If a company has a public competitor/comparable, you may be able to use comparables to estimate value of the company. Personally, if your only reason to take the job is how quickly you can make a buck through IPO flip, you most likely will be very disappointed.
seems like it might be about to IPO
Seems by whom? What is the likelihood of IPO happening?
Every startup I have talked to in past 4 years, claims they are going for IPO within a year or two. Not a single company among them have done the IPO yet. I personally see claims of impending IPO just a recruiting tool. Any IPO claims need to be discounted heavily. IPO is not easy to pull off! M&A is much more likely than IPO.
under some circumstances, could be a large portion of my compensation
What circumstances and the likelihood of those circumstances occurring? You need to list each circumstance, and its probability of happening, and the outcome.
If I end up considering the equity as worth $0, like some people recommend doing for jobs in startups, taking the job would be a huge pay cut from my current position, and I would not accept the job offer.
That is the baseline you should start with. Do a probabilistic analysis, the likelihood of different event, expected outcome and benefit to you. If you can't come up with a highly probable scenario, I wouldn't assume anything beyond baseline scenario to play out.
The stock prices at all rounds of funding, both the most recent and all previous ones
Do you have actual number of shares at each funding round or how much the company was valued at each funding round? It might be useful in valuing the company if it is really going for IPO. As investors expect M&A to be more likely event, most will have some sort of liquidation preferences attached to funding. So outcome for you in case of M&A is much more diluted.
The revenue growth since the last round of funding
Do you have actual revenue numbers or just claims by someone of revenue growth? If you have actual revenue numbers they will be useful in valuing the company, revenue growth not so much. Most of the time, these growth claims are inflated so will need to heavily discount such claims.
Employees are telling me that internal transparency about revenue has gone down recently, which they interpret as a sign of approaching an IPO
Do these employees have direct exposure/visibility to sales and revenue pipeline? How close are these employees to the pipelines where they can see the actual product, sales and revenue flow? More visibility, more believable unless they are working on convincing you of something. I have never met a Sales VP who claimed sales are not going that well.
Has company retained investment banker yet or do employees see suit-types (bankers, accounting, lawyers) showing up and hanging in the office or CEO making frequent trips to Investment Banking companies and locations? These are better indicators of IPOs. The pessimistic interpretation of reduced transparency will be that company is struggling to generate revenues and revenue growth and is in trouble.
For a complete picture of revenue and revenue growth you will need to talk to different internal groups that touch customers such as sales, marketing, service, account receivables, finance.
High-level employees have given me their personal estimates of the value of the stock today. Some of those estimates are too low for me to want to accept the job offer, some are high enough that I would want to accept the job offer.
The ones giving you the low estimates are likely to be more realistic unless personally they are impacted negatively by you taking the job. I will suggest to take some of them out of the office individually for lunch or coffee and see if you can dig in to the basis of their estimates and motives. I will ignore the ones giving high estimates, they are optimistically blind. Any employees with the midrange estimates who are closer to customers may be worth talking to.