Generally I tend to wonder what they want. What are they trying to "get from me"?
When I know that someone is giving me a sincere compliment over something I am proud of myself, like my cooking :) I believe them & say thanks. I feel honored & humbled that they felt compelled enough to compliment.
When it's something superficial like my appearance... I look for what they are trying to sell me or what favor they may want. "Flattery will get you everywhere".... only not when the other person is keen enough to know it's just a tactic.