The three basic tenets of information security and assurance are confidentiality, integrity, and availability, also known as CIA.
Confidentiality provides that information is only accessible to intended parties. Integrity ensures that the information has not been modified. Availability implies that information is available when it is needed.
Applying the concepts of CIA to cloud computing environments at a cursory level, these are a few questions you should ask your cloud computing provider:
From a confidentiality perspective, what controls are in place to ensure that other cloud customers can't view