You can check your milestones by accessing the "Others" menu, located at the top of the client, as suggested by the image on the right.
Some milestones can also grant bonuses, such as HP% and/or MP%, exp%, %elite chance, and damage.
You can check if a milestone grants you a bonus or not by expanding the desired category in which your milestone fits. The bonus should appear next to its description.
Currently, the maximum milestone bonuses available are: