When you are done building a MPGuino, you may want to perform some quick tests on key points with an ohmmeter to help ensure it was built correctly and that it will not have an adverse effect on your vehicle.
Below are measurements from a working MPGuino. As long as your readings are within an order of magnitude then it is probably safe to connect it to the car.
The meter was on k-ohms, where the red lead was positive. The column on the left are the points to place the black lead and the row on top are the test points to place the red leads.