When you look at an asset page you should be able to see little tag of which group it is a part of. If not, the SQL to see what asset belong to a group is going to be as follows:
SELECT A.AssetName, A.IPAddress, AG.AssetGroup
FROM tblAssets A, tblAssetGroups AG, tblAssetGroupLink GL
WHERE A.AssetID=GL.AssetID AND GL.AssetGroupID=AG.AssetGroupID AND GL.AssetGroupID!=0
This is assuming you do not want the 'Default Group'
If you want to know the groups of a specific asset:
SELECT A.AssetNAme, AG.AssetGroup
FROM tblAssets A, tblAssetGroups AG, tblAssetGroupLink GL
WHERE A.AssetID=GL.AssetID AND GL.AssetGroupID=AG.AssetGroupID AND A.AssetID=XXX
Where you replace the AssetID with your criteria. If you want to search by name for example, replace 'A.AssetID=XXX' with 'A.AssetName=XYZ'