terminals 85 and 86

No provided it doesn't have a built in diode.

Except for diodes, they are equivalent. If spark-quenching/suppression diodes are fitted, the convention is that #86 is the +ve and #85 the -ve side of the relay's coil. If reversed, the diode will short +12V to gnd until the diode blows, else the feeding circuit or fuse blows. If it's a snubber/quenching resistor rather than a diode, then it won't matter - they are non-polar like the coil. A relay should indicate a diode or resistor (rectangle) if they are included internally.

It's tricky - a voltage divider is probably best... IE - if connected with a resistor of similar value (to the coil - probably between ~80 to ~200 Ohms) and place across 12V, in one direction the diode will short the coil hence reducing the coil's voltage to about 0.6 volts, or conversely, brightening the bulb if a test lamp is used - ie, a 3W bulb is about 60 Ohms. But relays with diodes should be marked...