Find none ASCII characters

$string = "In mathematics, summation (capital Greek sigma symbol: ∑) is the addition of a sequence of numbers."
$string.ToCharArray() | ForEach-Object{
    $char = $_
    try{$ascii = [byte][char]$char}
    catch{"$char - None ascii character" }
}
Details
Breaks the string into a char array and attempts is convert the char back to it ASCII value.
If it fails to convert then it is not a standard ASCII character.
Output Example
PS C:\> ∑ - None ascii character
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