Customer-analyst interviews are considered among the most effective means to perform requirements elicitation. However, during these interviews, ambiguity can hamper commu- nication between customer and requirements analyst. Ambiguity is particularly dangerous in those cases in which the analyst misunderstands some linguistic expression of the customer, with- out being aware of the misunderstanding. On the other hand, if the analyst is able to detect ambiguous situations, this has been shown to help him/her in disclosing tacit knowledge. Indeed, the occurrence of an ambiguity might reveal the presence of unexpressed, system-relevant knowledge that needs to be elicited. Therefore, for the requirements elicitation interview to succeed, it is important for the analyst not to overlook ambiguities. To support the ambiguity-awareness of the requirements ana- lyst, this paper aims to provide a set of cues that can be identified in the linguistic expressions of the customer, and that typically lead to ambiguity. To this end, we performed 34 customer- analyst interviews, and we isolated the speech fragments that caused the ambiguity. Based on the analysis of these fragments, and leveraging the previous literature on ambiguity in written requirements, we identified a set of cues that can be used by requirements analysts as a reference handbook to detect ambiguities.