Brian Gaff (Sofa 2) has brought this to us :
I'd imagine one could design a circuit with a two colour led, where its red
one way and green the other and use an op amp and a low value resistor as a
shunt and measure the polarity with the amp. Crude, but should work, but
might need to be set up.

A pair of back to back LED's, with the centre point connected to the
point of polarity interest, and either end connected one to +ve, other
to -ve makes a good simple indicator. A bypass diode would be needed in
parallel with each LED, plus resistors to limit current.