Testing an automotive relay with a diode
You apparently have a relay with integrated diode accross the coil in the same unit.  If this is really the case, you should say so explicitly.  The term "automotive relay" by itself doesn't specify this.  It only means a relay intended for automobile use.

The diode will limit the voltage drop in one direction.  Use a 12 V supply (can be the 12 V from your car) with about 200 &Omega; in series.  Briefly connect this to the relay coil in each direction and measure the voltage accross the coil.  If you get over 1 V in one orientation and under 1 V in the other, then the diode is working.  If you get less than 1 V both ways, use a smaller resistor and repeat until you do get over 1 V in one direction.

Keep the power dissipation of the resistor in mind.  200 &Omega; directly accross 12 V would dissipate 12V2 / 200&Omega; = 720 mW.  A 1/2 W resistor can take that for a short time, but don't keep it connected for more than a second or so to get the voltmeter reading.  Also don't hold the resistor directly with your hand.  It will get hot enough to hurt quickly.  The best would be to use a resistor rated for the power, or multiple smaller resistors to effectively get one higher power resistor.