Some products have alarm function by using transistor output type.
Most common type is "Open collector" output and it can be used to detect operation of power supply.
Open collector behaves like switch that is either connected to signal ground or disconnected.
External voltage source is required to pull-up the signal voltage.
Current limiting resistor is required not to exceed maximum collector current which is shown in instruction manual.

Fig.1 Internal circuit of alarm function

Response time to become "High", depends on the product and condition.
Isolation of alarm signal depends on the product and please refer to instruction manual for detail.
Some products have AUX and AUX can be used as external voltage source.
Output current of AUX shall be within the rated current of AUX.