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Ralph Mowery

In article ,
says...

My second thought was if he uses 2 LEDs in parallel, and they're both above 5 volts, then they're both conducting. How much current goes through the forward biased one? enough to give a bright light, or maybe dim enough he thinks the circuit is dead and he burns up his new red sports car.

You put them so one diodes anode is connected to the cathode of the
other. Only one will lite at a time on DC.

Best little handy dandy checker I ever had is one made by Fluke. I
think it is a T2. Has about 10 or 12 LEDs in it and 2 leads. You just
hook the leads to anything you want to check. If open, nothing hapens.
if low resistance it beeps and a led lights up. Anything over about 3
volts DC and it gives an indication of power and direction. Good to
about 200 or 250 volts dc. When used on ac, it starts somewhat higher
and goes to atleast 600 volts. The leds inbetween nothing and mix give
a rough indication of more common voltges such as 120,220 480 and 600
volts AC. It starts at 6, 12, 24 and I forget the others for DC.