If you can see her before and after pictures, you can clearly see that there are not any clear signs of plastic surgery. Most of the pictures are edited before they publish on internet or magazines. The software has the power to change the picture. So, all these pictures which we see on the internet are mostly edited.
Secondly, Dianna Agron is only 29 years old, which is pretty young age for a plastic surgery. Many celebrities go under the knife to get different plastic surgery procedures. The most common procedures among celebrities are breast implants, nose job and butt implants. If you see the Kardashian family, you can see that every one of them has plastic surgery procedures.