Hi,
     A RG58 cabled network requires termination at both
ends, 50 ohms each. This results in a parallel circuit,
which should measure 25 ohms plus/minus 10%. This network
topology is designed to accommodate 30 nodes over 185 meters
at no less than 1/2(half) a meter apart. Testing such
without the high priced test gear, involves measuring at the
t's for 25 or so ohms, then man-handling the suspect t's
while watching the meter. If  only one node is affected,
that's where to start looking. I've had t's come apart in my
hands, the straight through section maintains the circuit
but the tap into the node is broken. Short and open circuits
halt all network traffic, cracks in shielding, cables
running to close to high power sources usually show up as
increased data errors.
      As long as you have a length of cable(greater than 1/2
a meter) terminated at both ends you have a network, with or
without devices connected.

                                            Ted