Such lines of evidence include statistical analyses of the internal structure of the survey including the relationships between responses to different survey items.
They also include relationships between the survey and measures of other constructs.
That is, to a layperson, does it look like it will measure what it is intended to measure?
In our example, would the people administering and taking the questionnaire think it a valid measure of depression?
You may use any of these scales at no cost without permission.