In the last week, I have looked for multi-part features a couple of times. Today, I was looking for multi-part polygons after dealing with the fall-out of a case of Clip Gone Wild as shown below.

I have not found a way to write a query to find these but Field Calculator does allow you to calculate a field’s value to the number of parts.

Using the Python parser, just write the formula (note that case matters): !shape!.partCount