Navionics has what they call webapp which will show you exactly what will be on a navionics card. It will show you what lakes it has plus what details it has of those lakes.
If you press the little icon in bottom left of the screen and switch to sonar charts you will see more detailed information(on some lakes anyways). This is information you can download from the Navionics website. After you download the information you have to change a setting on your fish finder to view this data, I don't know if it works on all fish finders or not.
https://webapp.navionics.com/?lang=e...key=cgjfIb_qtT
As walleyedude mentioned you can use genesis maps if you have a Lowrance unit and the maps on there are far more accurate but coverage is limited especially up north. Unless you have a fish finder with dual card readers you can only use one or the other(Navionics or Genesis Maps).