You should be able to look at box 1 of the 1099R for your gross distributions. If you have multiple 1099Rs, you would need to add them together to get your total distributions.
The important number is the amount in box 2a. This is taxable portion of your distribution. If you made any after tax or non deductible contributions to your retirement plan or IRA, the box 2a number will be different. If you rolled your IRA or retirement plan over to another account, your box 2a will be blank.
Your plan administrator will be in the upper right hand corner under payor. This may not be who you have your plan through if your employer uses a 3rd party plan administrator. If there is information under the Account number field. This may help you out. Otherwise contact the 3rd Party plan administrator at the number on the 1099R.
....or are you looking at a follow-up page after the main form?
IF so, which state are you living in ? Some states have a whole list of "sources" referring to Public pensions or Military pensions ...or.. NY & PA have a whole long list of "sources", for which...if you don't now, you'd have to ask the issuer by calling them if you don't really know