1-Try to include questions that check the consistency and validity of the respondent's answers.
For example, an employee survey can ask:
Which division of the company do you work in?
A later question can ask:
Where are you based?
When the data is analyzed responses can be filtered for nonexistent combination of divisions and geographical locations.
2-Always track the time spent by each respondent to answer the survey. Respondents that answer a survey very quickly may simply select options randomly. These responses should be excluded from the reports and the data analysis.