Plug in tester for Trailer Electricals.

Look in the ads in Caravan mags There used to be a connector with LED's built in to tell you if all is correct.

Otherwise a Battery World shop should know the answer.

Try Google, Whitworth's Marine have several about $12.00

Try these guys, Trailer Vision
Dennis has been building them for years. A Very approachable guy.

BCF, Whitworths Marine, and no doubt others, sell these >>>

KT Cable Accessories

I built our own from a few bits and pieces from DSE but it is not mobile. I live in a retirement village where the biggest past time is caravanning and we have about 70+ caravans here.
The device that I made is in two divisions. The first part is for testing lights and wiring for caravans and incorporates a 12v battery a series of switches and lights.
The second part plugs into a vehicles trailer plug and checks the wiring on the vehicle.
This machine is not portable but is on a trolley and can be wheeled about by us old codgers