Delete free apps from Play Store library
I suggest you use both a PC and your mobile for this solution. 

Using your PC, visit your Play Store's order history. It would list only those apps you have bought so far. Now, in your Play Store app, go to Account (rightwards swipe from the left edge) &rarr; Library.

Compare the two lists. The ones which would not match would be the free ones. 

This is not user-friendly but should get the job done nonetheless.