Cloud computing changed recently business view regarding their Information System through an on-demand provisioning of computing resources. Recent discussions about data security requirements in cloud computing environment tend to conflict with other requirement including usability and economic. In hy-brid clouds that combine private and public clouds usage, private clouds are able both to externalize resources and invoke services from public cloud when needed. However, in such specific inter-cloud environment risks arise. Indeed, private clouds aren't sufficiently assured about how credible is the data computed by these resources they entrusted. This is due to clouds autonomy preservation, difference in control policy definitions and lack of transparency in clouds. In this position paper, we tend to propose an approach to help private cloud selecting a trustworthy public cloud service. The idea consists in a trust manager as a service that bases the decision-making on the private cloud past invocation analysis