I am currently building a survey to use to perform a conjoint analysis and would like to include a test to show that people are actually considering between the options presented rather than just selecting them at random. Options I have seen suggested include providing a choice where one option is the obvious favourite, providing the same choice twice to see if they make the same selection, or including a timer to see how long someone spends considering the options. 

Does anyone have any experience including such tests in their design? Any advice would be appreciated