Akka, testing integrations with external systems

I have an Akka (Scala) project where all actors are integration of external services (Trello, Evernote, ...). My system works but it grows and I fell uncomfortable because I does not have any tests.

My question may not be related to Akka but : what are the techniques used to test a system that has integrations with external systems ? I'm thinking to setup WireMock but (here Akka come in the game) there may be some tools that are integrated into the Akka testkit.

Thanks