Measuring Subjective Beliefs
you can do a belief elicitation.
See more on this here:


Rauhut, H., 2013. Beliefs about lying and spreading of dishonesty: Undetected lies and their constructive and destructive social dynamics in dice experiments. PloS one, 8(11), p.e77878.


and here:


Gächter, S. and Renner, E., 2010. The effects of (incentivized) belief elicitation in public goods experiments. Experimental Economics, 13(3), pp.364-377.


If you don't want to invite them into the lab, you can use Amazon mTurk and oTree.