I suppose you could check Nexus at https://repository.jboss.org/nexus/
Do a search for arquillian and you'll get a list of every artifact and every version. It's not ideal, but it's what I'm doing for now.
the thing is that Alpha5 is really the latest Arquillian release. Because in CR1, Arquillian was split so now we have different versions for Arquillian Core (CR4), Arquillian Containers (CR1), Arquillian Drone (CR1), etc.
As Hakon has suggested, using Nexus to check the version is really the best solution for the moment. However, I filed a JIRA https://issues.jboss.org/browse/ARQ-565 to get the JIRA updated