how can you tell if a person is a rationalist?

the answer is a little harsh
no one has to do anything in reality its only a thought experiment
not for the weak hearted

it depends on how he responds to issues that he is very attached to emotionally
like his parents children religion etc

there are some good questions to test same

1. if your best friend comes into your house asking you to let him hide at your place. tells you that the police is following him and he needs your help. he confesses his crime to you.
2. if two children are drowning. one of them is yours and you can save one. you know your kid is low on morality cognition and aesthetics and the other kid is more promising. who would you save?
3. tell him a choice of products. ask him what he would choose and why? dig deeper into his answers. see if he is makes a rational choice and goes by the benefits of the product or takes a emotional decision of choosing a Brand.