All the ways you suggested will work to some extent, but some sort of computer based device is liable to be best . RS485 uses 2 wire differential voltage signalling and will usually have enough drive to allow an extra receiver to be attached. "Monitor" is probably a better term than "hijacker" :-).
A computer will provide the most flexible result as you can analyse the data and also display in an oscilloscope type display on the computer screen if desired.
An oscilloscope will allow you to see that signals are being sent and the levels involved but will provide little other information. Some oscilloscopes may provide means to analyses data via an RS485 interface but that is not usually an oscilloscope based function.
Special purpose hardware can easily be built and you can buy analysers with RS485 interfaces - but in many cases it is simpler to use a general purpose computer with an RS485 interface. The actual interface to a computer could be an off the shelf commercial RS485 to USB converter or RS485 to RS232 converter or you could build something using a data acquistion system.
Many examples of RS485 to USB interfaces here:
DIY RS485 - USB interface from here - with PCB layout. The circuit is copied from the FTDI data sheet but he has added much comment and construction detail.
DIY RS232 / RS485 interface
Commercially available RS485 analyser
and here , same as this