ExoneraTor is a website hosted by The Tor Project that tells you whether a given IP address was a Tor relay.
The ExoneraTor service maintains a database of IP addresses that have been part of the Tor network. It answers the question whether there was a Tor relay running on a given IP address on a given date. ExoneraTor may store more than one IP address per relay if relays use a different IP address for exiting to the Internet than for registering in the Tor network, and it stores whether a relay permitted transit of Tor traffic to the open Internet at that time.
Figure: ExoneraTor: Tor Relay Check