This is a fairly important warning sign and really quite easy to spot once you can tell all the little ways in which you are being verbally abused.
In addition to saying things that are meant to be cruel and hurtful, either in public or in private, this can include degrading remarks or running down any accomplishments.
Often the abuser will tell you that you are 'stupid', could not manage without him/her. He/she may keep you up all night to 'sort this out once and for all' or even wake you at night to continue to verbally abuse you.
The abuser may even say kindly things to your face, but speak badly about you to friends and family.