Select object and add decimate modifier, decrease ratio and see how poly count changes.
Add object from another blender file:
Select File/Append, select the file containing the desired object, the file will be treated like a directory containing sub dirs like objects, mesh, etc. Go to the object directory and select the desired object to append.
Rotate view around selection:
By default blender rotate view around object origin, which may be far from the portion of the object you are interested in. In edit mode, you can rotate view around selected meshes instead of object origin. To enable this, enable Rotate Around Selection under File/User Preferences/Interface.
Fix non-manifold meshes:
Non-manifold meshes contain edges belonging to more than two triangles. They cause problem with decimate modifier therefore needs to be fixed. To do this, add decimate modifier to an object to be fixed and check if there is problem. Tab into edit mode, select half of the mesh and press p to separate the object into two. Check to see if the two new objects can be decimated. If not, continue dividing the object into smaller objects until there are only a few meshes in the problematic object. Do visual inspection of the problematic object and usually the edge belonging to more than two triangles will be obvious. Here is an example:
