You need to use AssetDatabase.GetAssetPath() and Directory.Exists() which is part of the System.IO library in C#.

At the top of your script include the System.IO library:

using System.IO;

Then check your selections with the following:

foreach (Object obj in Selection.objects) {
    string selectionPath = AssetDatabase.GetAssetPath(obj); // relative path
    if (Directory.Exists(selectionPath)) {
        // do something
    }
}

Note that selectionPath is a relative path but Directory.Exists() is smart enough to realize this and still use it.

I just had to do something similar when trying to find a path for new assets. It's a little hacky for your usage but will get the job done without having to guess. I start with the assumption that each item is a directory and ask AssetDatabase.GenerateUniqueAssetPath(path) to give me a valid, unique path for a file inside that directory. If the item actually is a directory then a path is returned; but if it's a file then an empty string is returned.

for (var item in Selection.objects) {
    var selpath = AssetDatabase.GetAssetPath(item);
    if (selpath == "")
        // not an asset
        continue;

    var dummypath = System.IO.Path.Combine(selpath, "fake.asset");
    var assetpath = AssetDatabase.GenerateUniqueAssetPath(dummypath);

    if (assetpath == "") {
        // couldn't generate a path, current asset must be a file
        Debug.Log("File: " + item.name);
    }
    else {
        Debug.Log("Directory: " + item.name);
    }
}

I just had the same problem. AssetDatabase.GetAssetPath(myObject) returns the folder path (or any other asset path). The real problem is to verify that it's a folder. I'm not sure but it seems that only folders are UnityEngine.Objects (everything in Unity is derived from it but a folder just have this type)

Just tried it that way:

foreach (UnityEngine.Object O in Selection.objects)
{
    if (O.GetType() == typeof(UnityEngine.Object))
        Debug.Log("Folder: " + AssetDatabase.GetAssetPath(O));
}

but unfortunately ".unity" files (scene files) are Objects too. Since folders can contain "." (dots) the only way to verify that it's a folder would be to build the complete path and use http://System.IO to check if it's a folder.