Como puedo determinar si un mapa es precioso?

How can I determine if a map is precious or valuable you ask... go to a museum curator or a collector's market and have someone appraise it for you.

If you want to know how precise it is, well, you need to find other maps to compare it to. Go on google earth and compare distances between landmarks on both maps. Find Atlases and compare distances between those landmarks. It will take a bit of leg work, but if the distances you measure are all equivalent (with only a tiny margin of error), then your map is precise.