This is a non-opinion based followup to a previous question.
What constitutes "good faith" in negotiations?
What behavior during negotiations is considered unethical?
What measures can a party take if they suspect their counterparties of being dishonest?
Can one clandestinely record the conversation? E.g., the suspected counterparty talks among themselves in a foreign language, and the one wants to ask his employee (so that the NDA, if any, is not an issue) to listen and translate.
Can one clandestinely use voice-based lie detection? Please leave aside the issues of VSA's accuracy - let us assume for the purposes of this question that VSA is 100% accurate.