looking at devices for testing wires to see which is positive, which is negative.
do i want a light pen, or a digital multimeter, or something else?
thanks
A simple multimeter would do the job and can be bought very cheap.
I got some multimeters for less than £3.00 ! not so long ago for classroom teaching. Would do the job for your purpose. But please don’t ever be tempted to use on mains as they do not comply with any regulations for such use. The likes of Draper sell automotive versions but you really only need a cheapie with a d.c. voltage range. It will indicate - 12v if connected the wrong way round. Amazon start around £6.00 now I see.
You need a simple multimeter, as our fellows above have suggested. My advice is to buy the best you can afford. Take note, a multimeter won’t tell you the actual polarity of any wire in a dc system directly. The display readings will however give you the information to work things out.
Please read up on how to connect your meter correctly to give you the readings that will help you in whatever your task is. Good luck and enjoy the adventure!
Guess it depends on the meter.
Mine shows “+” before the voltage reading if I have red to +ve wire and black to -ve wire
It also shows “-” before the voltage reading if I have them reversed
Unless you want to get really exotic this is all you need.
The wires all contain 12v at some point, 0v is any point on the body.
thank you all, i had a feeling the basic see-thru screwdriver pen variety would be enough (thats just how they look to me). although i do like them snazzy digital doodads. 
Multimeter. Continuity tests are your friend for tracing circuits.
I’ve never come across one which doesn’t on d.c. But there’s always a first I guess. Different story for a.c. of course. If you connect one end to negative/ earth /chasssis, (the black / common on a multimeter) and probe with the red wire it will show 12v ish if contacts something live. If you swap the leads it will show -12v ish as mentioned. Easy enough to prove, just try it on a battery, any battery.
Kind regards
Marc
Right but my point was that you the user still have to know what your connection scheme is, the meter itself can’t tell you.