He looked so sincere that I could hardly judge whether he was telling the truth or falsehoods.
Examples (Wiktionary)
Mandarin
外来的考察团先生们在他们集合了各种材料，加上他们“想了一想”之后，他们就能够作出“共产党的抗日民族统一战线的政策是彻底的、诚恳的和真实的”这样一个判断了。
外來的考察團先生們在他們集合了各種材料，加上他們“想了一想”之後，他們就能夠作出“共產黨的抗日民族統一戰線的政策是徹底的、誠懇的和真實的”這樣一個判斷了。
wài lái de kǎo chá tuán xiān sheng men zài tā men jí hé le gè zhǒng cái liào ， jiā shàng tā men “ xiǎng le yi xiǎng ” zhī hòu ， tā men jiù néng gòu zuò chū “ gòng chǎn dǎng de kàng rì mín zú tǒng yī zhàn xiàn de zhèng cè shì chè dǐ de ， chéng kěn de hé zhēn shí de ” zhè yàng yī ge pàn duàn le ．
When the members of the observation group have collected various data and, what is more, have “thought them over”, they are able to arrive at the judgement that “the Communist Party's policy of the National United Front Against Japan is thorough, sincere and genuine”.