If someone receives a portion of their former spouse’s retirement monthly, which form should be used to count it as annual income?

It should be reported on Form 1099-R.  Here's some resources you can review from TurboTax: When to Use Tax Form 1099-R: Distributions From Pensions, Annuities, Retirement, etc. and  What do all the codes in Box 7 of the 1099-R mean?

Do you get it directly from the retirement plan or does  your ex send it to you?  If it comes from the plan  you should be getting your own 1099R for it.

Enter a 1099R under
Federal Taxes
Wages & Income
Then scroll way down to Retirement Plans and Social Security
Then IRA, 401(k), Pension Plan Withdrawals (1099-R) – Click Start