Can I calculate how much of my gross distribution was RMD?

Most RMD's were suspended for 2020.  If yours does not qualify for that, though, the financial institution can tell you the RMD amount.  Also see:

IRA Required Minimum Distribution Worksheet (irs.gov)

Please see this answer by HelenaC.

 

Generally, this is calculated by each account administrator, for each retirement account you have. You can request the account administrator send you your Required Minimum Distribution (RMD) each year.  

To calculate your RMD:

1. Find your age in the IRS Uniform Lifetime Table click on the link below.
2. Locate the corresponding life expectancy factor.
3. Divide your retirement account balance as of December 31 of the prior year by your life expectancy factor.

Link to Estimate your required minimum distributions in retirement | Vanguard

Calculating RMDs for multiple retirement plans

If you have more than one retirement plan, you'll need to calculate the RMD of each plan separately. However, you may add the RMD amounts of all IRAs (including traditional, rollover, SIMPLE, and SEP-IRAs) and withdraw the total amount from any one or more of your IRAs. The same rules apply to 403(b) accounts.

For example, assume that you have three IRAs. Your RMDs are $2,000 from the first IRA; $1,000 from the second IRA; and $1,000 from the third IRA. If you wish, you can take $4,000 from any one or more of your IRAs to satisfy your RMD for the year.

If you have accounts in several 401(k) or other employer-sponsored plans, the IRS generally requires you to calculate a separate RMD for each retirement plan in which you participate and withdraw the appropriate distribution from each plan.