There are so many ways you could check your wiring. Basically what you are looking for is a voltage drop under load between the amp neg terminal and the battery neg post. The smaller the voltage drop the better the ground. You need to do this with the amp pushing a load and drawing some current, since voltage drop is proportional to current. If the current draw is very small, the voltage difference will probably be smaller than your DMM can measure. As you increase the amp draw the voltage drop will move into the measurable range and give you a good indication of any losses that you are seeing from your ground point.
To measure this, set the DMM to VDC and measure the difference between the neg amp terminal and the neg post on the battery. The pos lead goes to the amp neg and the neg lead goes to the batt neg.