To access Inspect, you must be signed in to InVision V7, and you must also have edit access for the document. To learn more about permission settings for InVision V7, check out this article: InVision V7: What are the permission settings for documents?
This article provides answers for InVision V7. If you’re on InVision V6, read this article instead. Not sure which version you're using? Find out now.