Need to identify a common wire on my old HVAC
Whenever a furnace has a 24V low voltage transformer, the two 24V terminals are R and C.   

If you can't find any other way to do it, pick one of these two low-voltage terminals to use for C.  

Worst case, you picked R by mistake, and the thermostat will see 0 volts between R and C, since you are actually bringing it R and R.    In that case, use the other transformer terminal.  

On your furnace, the low voltage transformer is in the smaller schematic to the middle left.  Within that drawing, it shows the transformer in the middle.  The R wire snakes through a limit switch to the thermostat connection terminal block on the lower left.   Your furnace does not bring C out to that terminal block.  You'll have to "go get it".