Can it be determined from beyond visual range whether an unknown aircraft is armed or not?
Only if the airplane wants you to know it is armed.

It has several ways at its discretion:


If it has a powerful radar, it can switch it on at altitude and announce its presence to other aircraft up to 150 km away. 
It can send IFF squawks which are reserved for military aircraft. Note that IFF is used by both military and civilian aircraft.


Both methods are only indirectly indicating whether an aircraft is armed. The armament itself will not show up at BVR distances. In the end, if you want to be sure, you have to move some sensors close to the aircraft if the weapons are carried externally, or even wait until they are deployed when they are carried internally. Especially many small aircraft are used both in a civilian and a military role.

A BVR radar is a dead giveaway for a military aircraft - such equipment is not fitted with civilian aircraft, and removed or disabled when an ex-military aircraft is converted to civilian use. Similarly, supersonic speed, which can reliably be detected with Doppler radars, is a telltale sign of military aircraft approaching.

Transport aircraft are much harder to distinguish from bombers on the basis of radar signals alone, and in the end you cannot exclude that the hostile aircraft tries to spoof your equipment into classifying it as a civilian aircraft.