You will be able to check your Class Buffs through the [Misc. Menu] > [Class Buff] located in the upper right corner of the screen. 

You will also be able to check your Class Buffs through the clock icon located in the upper left corner of the screen.

※ Class buffs are unlocked when the points of a class reaches 40 points. You will be able to obtain up to max 55 points per Hero when strengthening a Hero to his/her maximum potential (T5 Lv.100). After a Class Buff has been unlocked, you will be able to check it through the Class Buff menu, and you will receive a message when obtaining Class points.