You can contact your pension plan administrator and ask what your RMD is. You can also use IRS Publication 590-B to find your future life expectancy in years from the appropriate table contained in the publication. You divide the value of your pension plan at the end of last year by your life expectancy to determine your required minimum distribution.
Why are you asking? At the beginning of the year your IRA or 401K plan should send you the amount you need to take each year. Or you can log into your account and see the RMD amount you need to take. Or did you get a 1099R for a pension?
For Pension it's whatever they send you. Say all of it is the RMD. If you have a RMD requirement it will ask you if it is the RMD. Say yes and enter the amount as the RMD amount. The pension doesn't need to calculate any RMD. Anything your pension pays you is considered to be the RMD. Traditional pensions automatically fulfill the rules of an RMD. So just enter the same amount in box 1 for the RMD for each 1099R.