All you need is a spare motor and LED or light bulb. There have been circuits on several sites showing how to build your own decoder tester. Do some Google searching for "decoder tester" and see what turns up.
Loy's Toys has been out of business for several years, but other companies are still making and selling his decoder tester.
I have one of the Loy's Toys decoder testers, but it is hardly ever used. I keep it at the club to use as a last resort after other testing methods such as a decoder reset command have failed when other members have decoder problems.
You can just clip the wires to a motor and some lights, but be careful of shorts. Or you can build your own tester following the pattern of one of the many commercial ones - for light functions use some LEDs with a 1K resistor on each one, for the motor leads either hook up a spare motor or use a bicolor LED and resistor, in one direction with will light green and the other red.
One of the first thing I did when I started buying DCC equipment was build myself a decoder tester. I used it ONCE - to verify that it worked. It's buried in a box somewhere. I've installed dozens of decoders and never had a bad one out of the box. My DCC system has a seperate low-power program track - my 'testing' it to put the loco on the program track and verify I can read and write to it there before I put it on the full power main track. The power of the program track is too low to damage the decoder if there is a wiring mistake. Once it is known to work fine on the program track I test on the main.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.