Visual inspection of wheel hub
can I take my (front + rear) hubs apart while it is still attached to the spokes?


Yes, of course you can.


  is it good enough to visually inspect the hub axle, and decide whether it's safe to use based on that?


Well, there's not much else you can do, so yeah, I'd call it good enough. If there's any doubt, I'd just replace the axles.


  any other safety concern?



Cracks in the hub body, especially at the spoke holes.
Cracks in the rim.
Rim so badly out of true that it can't be fixed. Not really a safety thing, but it could make the wheels useless.