“How does one accurately check if their unit even needs the "Map Offset"...??”
About the only way to do this is if you observe that your boat’s position is shown in one location yet some fixed structure is shown in another. The example of the bridge is a good one to use as these usually don’t move around a lot.

“How does one then use this feature to adjust their map accurately...??”
I think that Eddie did a good job of explaining this.

“If I have waypoints saved in my unit that were saved while the map was possibly off...will my saved waypoints be inaccurate after setting the "Map Offset"...??”
No they will not as the coordinates will be the same. They will appear in a different location on the map though. Think of it this way: the map is just a background picture that can be moved around. Setting the Map Offset feature effectively moves the background map around. It does not change the coordinates of your waypoints.

One thing that was not mentioned is that on some of the lakes and rivers on the mapping cards we think of these as a single big map. They are not always one map and can be several combined together. This means that the map offset may not be needed on all areas of a lake and setting it for one area of the lake may make it off on another.