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A simple way to check if a charger is switched mode or transformer is to put it close to an AM radio, switched mode devises generate a considerable amount of interference.
To be technical, for a transformer coupled device the loss depends on what is called the leakage inductance of the primary coils of the transformer. This is essentially the amount of magnetic flux in the coil that is not connected into the secondary.
Switch mode supplies do often connect across the main indirectly, through a rectifier and capacitor.
The main advantage of a switched mode PSU is that it can cope with a varying input voltage of 250v to 110v.A transformer type PSU could be designed to cope with such a range but would have to generate twice the required output voltage when operating from a 250v input and incoperate an internal voltage reduction circuit an expensive and lossy system.