Treggen Mavious owns a ship that has items I own on it, I messaged him and he doesn't reply.
Is there a way to tell whether he's ignoring me or inactive?

What can I do to get my items back?

Easiest way is to look at his profile if its not been hidden. If its been a while since he last logged on, then hes probably inactive. otherwise, without finding the faction he could be in to determine that, its unlikely.

Treggen Mavious is neither active nor able to be contacted, nor will he be for the forseeable future.

It is likely your items will remain inaccessible until combat can provide access.

Ok, thanks a lot guys.