How do you know if a vacuum tube is going to suffer from AC hum or not?
Hum caused by the indirect heater system is due to the diode effect between the filament and the cathode. Modern tubes (ie post WW2) have a special insulation that prevent filament’s space charge to appear but it can't be completely eliminated. Since cathodes are often positive in regard to the filament, this sets the diode in the non blocking way hence creates an electron flow between heaters and cathode which causes hum if the filaments are heated with AC.

Fixing the potential of the heaters at +15V above the cathode instead of fixing it to the 0V (or ground) sets the resulting diode as non passing hence prevent any noise.

I do complete a bit my answer. Unless the signal amplified by the tube is in the range of mV (like a guitar speaker signal), the noise created by the diode effect with AC filaments is insignificant. It is not the case in mic amplifiers or RIAA stages.