You should know all 5 SOLID principles. If you don't, read about them on Uncle Bob's site above, or watch the SOLID Pluralsight videos by Steve Smith.
SRP - Single Responsibility Principle - A class should have one, and only one reason to change.
OCP - Open Closed Principle - You should be able to extend a class's behavior without modifying it.
LSP - Liskov Substitution Principle - Derived classes must be substitutable for their base classes.
ISP - Interface Segregation Principle - Make fine-grained interfaces that are client specific.
DIP - Dependency Inversion Principle - Depend on abstractions, not on concretions.\
✅ Figure: Good example - Code using SOLID principles
After you have identified and corrected the most obvious broad principle violations, you can start drilling into the code and looking for localized code breaches. ReSharper from JetBrains or JustCode from Telerik are invaluable tools once you get to this level.
Once you understand common design principles, look at common design patterns to help you follow them in your projects.