怎么判断是不是UTF8编码？

DELPHI不知道怎么判断文件或字符串是否是UTF8编码。
我看到一个VB写的

Function OnUtf8(ByVal byts() As Byte) As Boolean
Try
Dim i As Int32, AscN As Int32
Do While i <= UBound(byts)
If byts(i) < 128 Then
i += 1 'ascii字符
AscN += 1
ElseIf (byts(i) And &HE0) = &HC0 And (byts(i + 1) And &HC0) = &H80 Then
i += 2 '2个字节的utf8
ElseIf (byts(i) And &HF0) = &HE0 And (byts(i + 1) And &HC0) = &H80 And (byts(i + 2) And &HC0) = &H80 Then
i += 3 '3个字节的utf8
Else
Return False
End If
Loop

断可以不要,当全部是ascii字符时,被划分到哪种编码
If AscN = byts.Length Then
Return False
Else
Return True
End If
Catch ex As Exception
'MsgBox(ex.Message)
End Try
End Function