How do you get around the fact that users aren’t always completely honest when providing feedback for fear of offending you?
You can’t completely avoid social desirability bias. Interestingly, the effect is even stronger when face-to-face, so simply being aware of the bias is very important.
Let the participant know that you want honest feedback. When you first brief them, say something along the lines of: “don’t worry about our feelings, we want your hear your honest reactions”.
Actions don’t lie. The actions that they take and the manner in which they react non-verbally are rarely padded or fake.
Phrase your questions carefully. If the participant senses that there is a desired positive reply, they will want to give that to you.
Emphasize the usefulness of “negative” feedback when the participant gives it. This will make them more comfortable with scrutiny.