EDIT / UPDATE: If you wish to also use this circuit for voltages LOWER then 12 v or so, you may need to install a bypass switch over the resistor and to be safe, include a 1 k resistor in series with the switch.
Step 1: Gathering the Parts
R1 = 11 K 1/4 W resistor
D1 = 1 Bright Green LED
D2 = 1 Bright Red LED
D4, D5, D6, D7 = 4 1N4004 (or you can substitute 1N4005, 1N4007) Silicon Rectifier Diodes.
Misc: PC Board (optional), wire, case, Probes / Clips
Tools: soldering iron, needle nosed pliers, possibly hot glue and glue gun, etc.
Step 2: Basic Construction
The Schematic shows the basic connection of the parts.
Step 3: Seal It, or Box It
I merely sealed it in hot glue as pictured. It isn't very pretty, but it functions well.
Some tips as pictured, if you will be using insulated clips as I have, first thread the wire through the removed insulator as shown, and then solder / crimp it to the alligator clip.
If you plan on covering a large area with hot glue, it might be to your advantage to tape or cover in some ways, the device in case you need to use it outside in the heat, or inside repeatedly where it may get warmed by you hands or other source.
Step 4: Testing the Tester
The first picture shows that the Red wire attached to the positive portion of the PS plug, lights the green LED.
I then attached it to a known source directly to 115 v AC, and both LEDs lit, as would be expected.
Step 5: Final Step: Optional
The last picture shows a 9 volt battery hooked up with the polarity reversed, demonstrating that this works also.
EDIT: I have installed a switch. Since the design made the 11 k resistor out of a 10 k plus a 1 k, I simply soldered the switch to bypass the 10 k resistor when closed. Opened, the current flows through all 11 k.