Your mirrors need to give you a view about 200 yards back behind you on either side with no blind spots all the way to your side windows
One way to check is in a large parking lot, park out in an empty part at one end and have a helper walk out behind you on one side and back on the other. Use the center of the two parking spots beside you to act as your "lanes".
The other thing is can you back in at a 90 degrees and see your back end in your mirror. Typical of backing into a camping spot at right angles to the road. If you can't see your trailer in the mirror your gonna hit something eventually.