I am planning to install an arc fault breaker on a knob and tube wired circuit. I have installed the arc fault breakers on a sub panel that is wired with romex, which was easy to pair the same circuit hot and neutral together. Since I need to pair the hot and neutral together with the knob and tube, does anyone have any advice on how to identify the neutral that should be paired with the hot? It is all like spaghetti coming into the panel.

My suggestion would be to plug something in an outlet or turn on a light on that circuit.

Turn all breakers off. Remove a neutral wire, turn breaker for that circuit on. If light comes on that is not the neutral for that circuit. Eventually you will find that the light does not come on when breaker is turned back on. That would be the neutral for that circuit.

Keeping the knob and tube? Not really acceptable.

Typically knob and tube circuits share neutrals...
so I don't think an Arc fault breaker will even work here.
If you are attempting to improve the wiring method either
for yourself or a client, consider updating the knob and tube
to NM.