How to check a value is SPGroup or SPUser

SPList olist = oweb.Lists.TryGetList("Test_List");

if (olist != null)

{

SPListItemCollection ocoll = olist.Items;

foreach(SPItem oitem in ocoll)

{

SPFieldUserValueCollection GetAllGroup = new SPFieldUserValueCollection(oweb, Convert.ToString(oitem["SPGroups"]));

for (int i = 0; i < GetAllGroup.Count; i++)

{

SPFieldUserValue singlevalue = GetAllGroup[i];

if (singlevalue.User == null)   // If it is GROUP condition is true

{

SPGroup group = oweb.Groups[singlevalue.LookupValue];

foreach (SPUser user in group.Users)

{

if (ouser.Name == user.Name)

{

isuserval = true;

break;

}

}

}

else

{

if (ouser.Name == singlevalue.User.Name) // Checks User is Current Login User

{

isuserval = true;

break;

}

}

}

}

}