We identify several polar question forms in Sign Language of the Netherlands (NGT) through a production experiment in which we manipulate two types of biases: (i) the prior expectations of the person asking the question, and (ii) the evidence available in the immediate context of utterance. Our analysis in the present paper focuses on forms involving headshake. We find that in some cases headshake expresses negation, as expected, but in other cases it fulfils another function, namely, it is part of a sentence-final phrase either expressing uncertainty or signalling a request for a response from the addressee, or possibly both at the same time. We further observe that each question form has a distinct ‘bias profile’, indicating a certain combination of prior expectations and contextual evidence. Besides these empirical findings, our study also makes a methodological contribution: our experimental design could be used in future work to identify polar question forms with different bias profiles in sign languages other than NGT, as well as visual cues accompanying polar questions with different bias profiles in spoken languages.