The voluntary carbon market is an important building block in the fight against climate change. However, it is not trivial for consumers to verify whether carbon offset projects deliver what they promise. While technical solutions for measuring their impact are emerging, there is a lack of understanding of how to translate this data into interface designs that mediate the establishment of trust. With interaction between users and offset projects mainly happening online, it is critical to meet this design challenge. To this end, we designed and evaluated interfaces with varying trust cues for carbon offset projects in a randomized online experiment (n=244). Our results show that content design, particularly financial and forest-related quantitative data presented at the right detail level, increases the perceived trustworthiness, while images have no significant effect. We contribute the first specific guidance for interface designers for carbon offsets and discuss implications for interaction design.