I am going by the assumption of the worst case situation in which your doorbell receiver plugs into a wall outlet. In that case, yes, you should reverse the plug to check to see whether the power supply is transformerless.Thanks for your reply. Do you mean reversing polarity of the 240v. Sounds scary!! But if I have to I’ll do it
If your doorbell receiver is hard-wired into the household wiring then the only check you need to do is to measure the AC between the doorbell's output and earth. It is good if the voltage is not large enough to interfere with the operation of the interface being discussed here or to hurt you. If it is a large voltage, then need for isolation is indicated.