untitled

private static string CheckTag(string value)
{
    if (string.IsNullOrWhiteSpace(value))
        return "NONE";

    var tagPrefixes = new[]
    {
        "SMP_",
        "WKP_",
        "GEP_",
        "INP_"
    };

    foreach (var prefix in tagPrefixes)
    {
        if (value.Trim().StartsWith(
            prefix,
            StringComparison.OrdinalIgnoreCase))
        {
            return prefix;
        }
    }

    return "NONE";
}