Following points should be taken into consideration :
1. Look into all individual commits
2. look first into the files which have relevant changes.
3. We need to make sure that the PR's code should be woking as properly as per the new feature for which the PR is raised.
4. code should not affect any previously existing feature.
3. functions should not be tool long.
4. There should not be redundant code.
6. Variables and function names should be meaningful .
7. If the application contains unit tests than the test coverage should criteria should be met.
for more details please refer the following link :
What You Should Look Out For When You Review Pull Request