How do I know whether distribution of $25,000 from my Equitable account was a required minimum distribution?

RMD stands for Required Minimum Distribution. It's the minimum amount of money you're required to take out of your retirement plan once you turn 70 1/2 (Age 72 as of 2020).

RMD rules apply to all employer-sponsored retirement plans such as pensions, profit-sharing, 401(k), 403(b), and 457(b) plans, as well as Traditional IRAs and IRA-based plans such as SEPs, SARSEPs, and SIMPLE IRAs. However, RMDs are not required for Roth IRAs while the owner is still alive.

Your 1099-R issuer is required to follow RMD rules and regulations, so if you received a distribution and you're at least 72, you can be almost certain you received an RMD. Check with your plan administrator if you're still not sure.