You are correct – the power transformer secondary (high voltage) winding is not centre-tapped. Note that the low end of the winding is connected directly to ground, so all you need to do is to check the resistance between the plate of the rectifier (which is where the hot end of the winding is connected) and ground.
This will check for an open or high-resistance circuit, but not for a short circuit, unless you know what the secondary winding DC resistance is supposed to be.
If the rectified tube is faulty, it might have leakage to ground from the plate, so it’s probably a good idea to remove the tube before testing, unless you’ve already tested all the pins.