Differentiating between free PS+ games and purchased games

The only way to check if a game on your PS4 is a Plus game is to see if the game has an expiring licence. Plus games have a licence ending at the same time as your Plus subscription, while purchased games don't.

To see if your game has an expiring licence select the game tile, press the options button and select "Information". A PS Plus game will show lines for "Valid Period (Start)/(End)" and "Remaining Time", while purchased games don't have these lines.

Now you can go to your Library and PS+ games will be listed there in the separate group: