How do I calculate long term capital gain on a 1099-DIV cash liquidation distribution

Yes, you have to report a liquidation distribution as a sale, and you will need to know your basis. For a liquidation of stock, you should be able to get it from your broker. Follow this link for more about basis. 

You can enter this type of transaction in TurboTax. Here's how to do it:
 

1. Sign in to your TurboTax account.
2. Select Wages & Income, then Edit.
3. Select Start or Update to the right of Stocks, Cryptocurrency, Bonds, Mutual Funds, Other.
4. Answer the first question Yes, or if you already sales here,  select Add more sales.
5. Select the Other tile, then Continue.
6. Enter the information on the next few screens according to your situation. 
7. Enter Liquidation distribution as the name, then select Continue.
8. For type, select Stock, unless one of the other descriptors is more accurate. 
9. Answer the rest of the questions on the screen according to your situation, then select Continue.
10. Select None of these apply, then Continue.
11. Answer the last question based on the length of ownership. 

This will create a Form 8949 on your tax return and Schedule D. The transaction will show as either C, or F, depending on how long you owned the asset before the liquidation.