AssetDatabase.IsForeignAsset

Description

Determines whether the Asset is a foreign Asset.

A foreign Asset is an external file that you import into your Unity Project (for example, a .png Texture file is a foreign Asset).
Upon importing a foreign Asset, Unity will produce a serialized representation of the data in the Project's Library folder. This is done when the Asset is first added to the Project and when the foreign Asset is updated externally to Unity.

Additional resources: AssetDatabase.IsNativeAsset.

using UnityEditor;
using UnityEngine;

public class AssetDatabaseExamples : MonoBehaviour
{
    [MenuItem("AssetDatabase/Find Foreign Assets")]
    static void FindForeignAssets()
    {
        //Find all foreign assets
        foreach (var guid in AssetDatabase.FindAssets("",new []{"Assets"}))
        {
            var path = AssetDatabase.GUIDToAssetPath(guid);
            var asset = AssetDatabase.LoadMainAssetAtPath(path);
            var assetIsForeign = AssetDatabase.IsForeignAsset(asset);
            if(assetIsForeign)
                Debug.Log(asset);
        }
    }
}