If I inherit a traditional IRA (as a non-spouse) as a lump-sum distribution, do I need to know if any of that amount was an RMD?

Since you took out the balance of the account, the amount that was an RMD, isn't really important. You can answer that all of it was an RMD, or you can say part of it was an RMD, that is more accurate, but then you need to enter your RMD amount.

• The trustee of the account should have told you how much you have to take out, that would be the RMD, but if you don't have that information you can just estimate the amount. 

The RMD is based on your age. If you are 60 it is about 4% of the account value. If you are younger it is less, if you are older it is more. There are RMD calculators on line if you want to be precise. 

One thing you should know is that, no matter what you answer, the information is only used to see if you owe penalty. These numbers don't appear on your tax return.

Since you took the full balance of the account you do not owe a penalty. 

Source: Expert JulieS

What TurboTax is really asking is how much of the distribution is ineligible for rollover.  Since none of a distribution paid to a non-spouse beneficiary is eligible for rollover, all of it is ineligible for rollover.  With no rollover, it doesn't matter what answer you provide to the question asking how much is RMD.  In fact, TurboTax is programmed to know that a non-spouse beneficiary is not allowed to roll over a distribution, so it doesn't even need to be asking this question under these circumstances.