I've created a project page on Github for amcheck, a tool for verifying the logical consistency of PostgreSQL B-Tree indexes:
The tool is primarily useful for detecting index corruption in production database systems. It can do this with low overhead; most verification requires only a non-disruptive lock on the index as it is verified. The strength of the lock taken on an index as it is verified matches that of simple SELECT statements (unless the highest level of verification is requested). The locking involved will generally not block concurrent reads or writes, and will not prevent VACUUM from running concurrently.