You should be frank with the person and tell him/her that you do not feel or think his/her opinion is genuine. If the person is telling you anything just to please you, he/she is not your friend. This is because friends do not mind what you think of them but act to their best, so that you can cope with their personality.
sometimes i try to turn it around somehow,say i ask 'these blue jeans make my rear look fat?" thay say 'no' and i think they are lying, so theni say " i think i look fatter in this pair,but your saying i look fatter in the black jeans?" so i corner them...they have to answer either the blue or black make me look bigger, it doesnt allow them just to stay with one lie,they have to choose and explain themselves