How to find out the IPO lockup period?

I don't know about the general case, but as an example, the S-1 of Medallia (MDLA) contains a section titled "Lock-Up and Market Standoff Agreements" (emphasis mine):

We will agree that we will not (i) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase or otherwise dispose of, directly or indirectly, [...] any shares of our common stock or securities convertible into or exchangeable or exercisable for any shares of our common stock [...] for a period of 180 days after the date of this prospectus, other than the shares of our common stock to be sold hereunder and certain other exceptions.

and

Our directors, our executive officers, the selling stockholders and holders of substantially all of our capital stock and securities convertible into our capital stock have entered or will enter into lock-up agreements with the underwriters prior to the commencement of this offering pursuant to which each of these persons or entities, with limited exceptions, for a period of 180 days after the date of this prospectus, may not, without the prior written consent of BofA Securities, Inc. and Wells Fargo Securities, LLC, (1) offer, pledge, sell, contract to sell [...]

and (I assume the following is about incentive stock options):

In addition, we have entered into agreements with our executive officers, directors and holders of our capital stock and securities convertible into or exchangeable for our capital stock, including our IRA and our standard form of option agreement, that contain certain market standoff provisions under which they have agreed that, subject to certain exceptions, for a period of 180 days after the date of this prospectus, they will not, without our prior written consent, dispose of or hedge any shares or any securities convertible into or exchangeable for shares of our common stock.

There was also an early release of 10% of the stock, the rationale being that the general release happened during a blackout window (and even though that would prevent only current employees from trading, past employees were also allowed to sell 10% early):

if (i) at least 120 days have elapsed since the date of this prospectus and (ii) the lock-up period is scheduled to end during a broadly applicable and regularly scheduled period during which trading in our securities would not be permitted under our insider trading policy, or a blackout period, or within five trading days prior to a blackout period, such lock-up period will end with respect to 10% of the securities subject to such lock-up agreements 15 trading days prior to the commencement of the blackout period

(Disclaimer: I worked at Medallia before their IPO, and still hold shares there.)