If you have a multimeter, you can test the defrost heater for proper resistance.

Set the multimeter to the ohm setting.

Trace the two wires of the defrost heater to its connector. Disconnect the connector by squeezing the tab and pulling it straight out.

In this case, the defrost fuse and the heater are wired in series, so your test will also show if the fuse is ok.

In the photos here, the orange wires are for the defrost thermistor, not the defrost heater. The heater wires will usually be larger sized.