Use question with trap - insert a question with a trap in the questionnaire. The trap questions includes an instruction to choose a specific answer.
Source: Decieux, J. P. P., Mergener, A., Neufang, K. & Sischka, P. (2015). Higher response rates at the expense of validity? Consequences of the implementation of the ‘forced response‘ option within online surveys. Presented at General Online Research 2015, Germany.