Where should I connect the C wire in a Janitrol furnace?
It does look like it might be a common, but only way to know for sure is to either look at a schematic of the board, or test it with a multi-meter. 

Using a multimeter, set to VAC (Volts AC) mode, you should read 24V between the R and C (or in your case, "24V") terminals. You'll probably have to at least partially pull out the connector currently on the R to get the multimeter probe on it.

For reference:


R = Constant 24V ac
W = Heating contact (the thermostat connects this to R to turn on heat)
G = Fan contact (the thermostat connects this to R to turn on fan)