What are the symptoms of a bad regulator rectifier?

The reliable symptoms are electrical, not sensory. Either the battery is not being charged, so voltage at the battery does not rise with engine speed, or it is being overcharged, so voltage climbs abnormally high and the battery becomes hot, swollen or gassy. Dim or flickering lights and a bike that runs progressively worse as the battery drains are downstream effects of the first case.

Can I test a regulator rectifier with a multimeter?

You can test the rectifying half using diode mode, checking that each path conducts one way and blocks the other. You cannot properly test the regulating half on the bench, because that function only operates with the system running. That is why the running voltage measurement at the battery is the test that decides it.

Why does my regulator get so hot?

Regulators dissipate surplus energy as heat by design, so warm to hot is normal. What is not normal is heat that has discoloured or melted the connector, or a unit hot enough that the surrounding wiring has suffered. Many machines mount the regulator in the airflow for exactly this reason, so a fairing change or an accessory that blocks airflow can contribute.

Will a bad regulator rectifier drain my battery when parked?

It can. A failed diode can allow current to leak back through the unit while the bike is switched off, which shows up as a parasitic draw. If your standing draw test points at the charging circuit, unplug the regulator and repeat the measurement. See testing for a parasitic draw.

Is an aftermarket regulator a good idea?

Quality varies widely. What matters more than brand is fixing the reason the original failed, and making sure the replacement has adequate cooling and a sound connector. A well-made unit fitted to a burnt connector will still fail.